CSS Backgrounds and Borders Module Level 3

Abstract

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc. This draft contains the features of CSS level 3 relating to borders and backgrounds. It includes and extends the functionality of CSS level 2 [CSS21], which builds on CSS level 1 [CSS1]. The main extensions compared to level 2 are borders consisting of images, boxes with multiple backgrounds, boxes with rounded corners and boxes with shadows.

This module replaces two earlier drafts: CSS3 Backgrounds and CSS3 Border.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document was produced by the CSS Working Group as a Candidate Recommendation.

A Candidate Recommendation is a document that has been widely reviewed and is ready for implementation. W3C encourages everybody to implement this specification and return comments to the (archived) public mailing list [email protected] (see instructions). When sending e-mail, please put the text “css3-background” in the subject, preferably like this: “[css3-background] …summary of comment…”

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The following features are at-risk and may be dropped at the end of the CR period if there has not been enough interest from implementers: ‘box-decoration-break’, animatability of ‘box-shadow’

The CSS WG maintains an issues list for this module. A test suite and implementation report for CSS Backgrounds and Borders Level 3 will be developed during the Candidate Recommendation phase, which will last a minimum of three months, and at least until 25 July 2012. See the section “CR exit criteria” for more details.

The changes to this specification since the previous drafts are listed in the “Changes” section.

Table of contents

• 1. Introduction
• 2. Values and Interactions
  • 2.1. Module Interactions
  • 2.2. Value Types
  • 2.3. Animated Values
• 3. Backgrounds
  • 3.1. Layering Multiple Background Images
  • 3.2. Base Color: the ‘background-color’ property
  • 3.3. Image Sources: the ‘background-image’ property
  • 3.4. Tiling Images: the ‘background-repeat’ property
  • 3.5. Affixing Images: the ‘background-attachment’ property
  • 3.6. Positioning Images: the ‘background-position’ property
  • 3.7. Painting Area: the ‘background-clip’ property
  • 3.8. Positioning Area: the ‘background-origin’ property
  • 3.9. Sizing Images: the ‘background-size’ property
  • 3.10. Backgrounds Shorthand: the ‘background’ property
  • 3.11. Backgrounds of Special Elements
• 4. Borders
  • 4.1. Line Colors: the ‘border-color’ properties
  • 4.2. Line Patterns: the ‘border-style’ properties
  • 4.3. Line Thickness: the ‘border-width’ properties
  • 4.4. Border Shorthand Properties
• 5. Rounded Corners
  • 5.1. Curve Radii: the ‘border-radius’ properties
  • 5.2. Corner Shaping
  • 5.3. Corner Clipping
  • 5.4. Color and Style Transitions
  • 5.5. Overlapping Curves
  • 5.6. Effect on Tables
• 6. Border Images
  • 6.1. Image Source: the ‘border-image-source’ property
  • 6.2. Image Slicing: the ‘border-image-slice’ property
  • 6.3. Drawing Areas: the ‘border-image-width’ property
  • 6.4. Edge Overhang: the ‘border-image-outset’ property
  • 6.5. Image Tiling: the ‘border-image-repeat’ property
  • 6.6. Drawing the Border Image
  • 6.7. Border Image Shorthand: the ‘border-image’ property
  • 6.8. Effect on Tables
• 7. Miscellaneous Effects
  • 7.1. Drawing at Breaks: the ‘box-decoration-break’ property
  • 7.2. Drop Shadows: the ‘box-shadow’ property
• 8. Definitions
  • 8.1. Glossary
  • 8.2. Conformance
  • 8.3. Levels
    • 8.3.1. Level 1
    • 8.3.2. Level 2
    • 8.3.3. Level 3
  • 8.4. CR Exit Criteria
• 9. Changes
  • 9.1. Changes since the 17 April 2012 Candidate Recommendation
  • 9.2. Changes since the 14 February 2012 “Last Call” Working Draft
  • 9.3. Changes Since the 15 February 2011 Candidate Recommendation
  • 9.4. Changes Since the 17 December 2009 Candidate Recommendation
• 10. Acknowledgments
• 11. References
  • Normative References
  • Other References
• Property Index
• Index

1. Introduction

This subsection is not normative.

When elements are rendered according to the CSS box model [CSS21], each element is either not displayed at all, or formatted as one or more rectangular boxes. Each box has a rectangular content area, a band of padding around the content, a border around the padding, and a margin outside the border. (The margin may actually be negative, but margins have no influence on the background and border.)

The properties of this module deal with the decoration of the border area and with the background of the content, padding and border areas. Additionally the box may be given a "drop-shadow" effect with the ‘box-shadow’ property.

If an element is broken into multiple boxes, ‘box-decoration-break’ defines how the borders and background are divided over the various boxes. (An element can result in more than one box if it is broken at the end of a line, at the end of a column or at the end of a page; and continued in the next line, column or page.)

The relative stacking order of backgrounds, borders, and shadows is given in this module. For how these layers interact with other rendered content, see Appendix E “Elaborate description of Stacking Contexts” in [CSS21].

2. Values and Interactions

This specification follows the CSS property definition conventions from [CSS21].

2.1. Module Interactions

This module replaces and extends the background and border features defined in [CSS21] sections 8.5 and 14.2.

All properties in this module apply to the ::first-letter pseudo-element. The background properties and border-radius properties also apply to the ::first-line pseudo-element. The UA may (but is not required to) apply the border-image or ‘box-shadow’ properties to ::first-line. The UA must not apply the border-color/style/width properties to ::first-line. [CSS21]

2.2. Value Types

The <image> value type is defined by this specification as <image> = <uri>. Other value types are defined in CSS Level 2 Revision 1 [CSS21]. Other CSS modules may expand the definitions of these value types: for example [CSS3COLOR], when combined with this module, expands the definition of the <color> value type as used in this specification. Similarly, [CSS3-IMAGES], when combined with this module, expands the definition of <image> as used in this specification.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

2.3. Animated Values

It is expected that CSS will include ways to animate transitions between styles. (The section “Animation of property types” of the CSS Transitions module [CSS3-TRANSITIONS] is expected to define how different kinds of values are interpolated during a transition.) In anticipation of that, this module includes a line “Animatable” for each property, which specifies if the property can be animated or not.

3. Backgrounds

Each box has a background layer that may be fully transparent (the default), or filled with a color and/or one or more images. The background properties specify what color (‘background-color’) and images (‘background-image’) to use, and how they are sized, positioned, tiled, etc.

The background properties are not inherited, but the parent box's background will shine through by default because of the initial ‘transparent’ value on ‘background-color’.

3.1. Layering Multiple Background Images

The background of a box can have multiple layers in CSS3. The number of layers is determined by the number of comma-separated values in the ‘background-image’ property. Note that a value of ‘none’ still creates a layer.

Each of the images is sized, positioned, and tiled according to the corresponding value in the other background properties. The lists are matched up from the first value: excess values at the end are not used. If a property doesn't have enough comma-separated values to match the number of layers, the UA must calculate its used value by repeating the list of values until there are enough.

background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left, bottom right;
background-origin: border-box, content-box;
background-repeat: no-repeat;

background-image: url(flower.png), url(ball.png), url(grass1.png);
background-position: center center, 20% 80%, top left;
background-origin: border-box, content-box, border-box;
background-repeat: no-repeat, no-repeat, no-repeat;

The first image in the list is the layer closest to the user, the next one is painted behind the first, and so on. The background color, if present, is painted below all of the other layers.

Note that the border-image properties can also define a background image, which, if present, is painted on top of the background created by the background properties.

3.2. Base Color: the ‘background-color’ property

This property sets the background color of an element. The color is drawn behind any background images.

h1 { background-color: #F00 } /* Sets background to red. */

The background color is clipped according to the ‘background-clip’ value associated with the bottom-most background image.

3.3. Image Sources: the ‘background-image’ property

This property sets the background image(s) of an element. Images are drawn with the first specified one on top (closest to the user) and each subsequent image behind the previous one. Where

<bg-image> = <image> | none

A value of ‘none’ counts as an image layer but draws nothing. An image that is empty (zero width or zero height), that fails to download, or that cannot be displayed (e.g., because it is not in a supported image format) likewise counts as a layer but draws nothing.

See the section “Layering multiple background images” for how ‘background-image’ interacts with other comma-separated background properties to form each background image layer.

When setting a background image, authors should also specify a ‘background-color’ that will preserve contrast with the text for when the image is unavailable.

For accessibility reasons, authors should not use background images as the sole method of conveying important information. See Web Content Accessibility Guideline F3 [WCAG20]. Images are not accessible in non-graphical presentations, and background images specifically might be turned off in high-contrast display modes.

Note that stylistic foreground images can be provided in CSS with the content property. (Semantically-important foreground images should be provided in the document markup, e.g. with the <img> tag in HTML.)

Media fragments can be used to display a portion of an image. The CSS Images module will provide fallback syntax for image formats and include additional controls for image display.

body { background-image: url("marble.svg") }
p { background-image: none }
div { background-image: url(tl.png), url(tr.png) }

Implementations may optimize by not downloading and drawing images that are not visible (e.g., because they are behind other, fully opaque images).

3.4. Tiling Images: the ‘background-repeat’ property

Specifies how background images are tiled after they have been sized and positioned. Where

<repeat-style> = repeat-x | repeat-y | [repeat | space | round | no-repeat]{1,2}

Single values for <repeat-style> have the following meanings:

‘repeat-x’

Computes to ‘repeat no-repeat’.

‘repeat-y’

Computes to ‘no-repeat repeat’.

‘repeat’

Computes to ‘repeat repeat’.

‘space’

Computes to ‘space space’

‘round’

Computes to ‘round round’

‘no-repeat’

Computes to ‘no-repeat no-repeat’

If a <repeat-style> value has two keywords, the first one is for the horizontal direction, the second for the vertical one, as follows:

‘repeat’

The image is repeated in this direction as often as needed to cover the background painting area.

‘space’

The image is repeated as often as will fit within the background positioning area without being clipped and then the images are spaced out to fill the area. The first and last images touch the edges of the area. If the background painting area is larger than the background positioning area, then the pattern repeats to fill the background painting area. The value of ‘background-position’ for this direction is ignored, unless there is not enough space for two copies of the image in this direction, in which case only one image is placed and ‘background-position’ determines its position in this direction.

‘round’

The image is repeated as often as will fit within the background positioning area. If it doesn't fit a whole number of times, it is rescaled so that it does. See the formula under ‘background-size’. If the background painting area is larger than the background positioning area, then the pattern repeats to fill the background painting area.

‘no-repeat’

The image is placed once and not repeated in this direction.

Unless one of the two keywords is ‘no-repeat’, the whole background painting area will be tiled, i.e., not just one vertical strip and one horizontal strip.

body {
  background: white url("pendant.png");
  background-repeat: repeat-y;
  background-position: center;
}

body {
  background-image: url(dot.png) white;
  background-repeat: space
}

See the section “Layering multiple background images” for how ‘background-repeat’ interacts with other comma-separated background properties to form each background image layer.

3.5. Affixing Images: the ‘background-attachment’ property

If background images are specified, this property specifies whether they are fixed with regard to the viewport (‘fixed’) or scroll along with the element (‘scroll’) or its contents (‘local’). The property's value is given as a comma-separated list of <attachment> keywords where

<attachment> = scroll | fixed | local

fixed

The background is fixed with regard to the viewport. In paged media where there is no viewport, a ‘fixed’ background is fixed with respect to the page box and therefore replicated on every page. Note that there is only one viewport per view. Even if an element has a scrolling mechanism (see the ‘overflow’ property [CSS21]), a ‘fixed’ background doesn't move with the element.

local

The background is fixed with regard to the element's contents: if the element has a scrolling mechanism, the background scrolls with the element's contents. In this case, the background behind the element's border (if any) scrolls as well, even though the border itself does not scroll with the contents. The UA may, however, treat the ‘border-box’ value of ‘background-clip’ as ‘padding-box’ in cases where ‘background-attachment’ is ‘local’ and the contents of the element scroll.

scroll

The background is fixed with regard to the element itself and does not scroll with its contents. (It is effectively attached to the element's border.)

Even if the image is fixed, it is still only visible when it is in the background painting area of the element or otherwise unclipped. (See “The backgrounds of special elements” for the cases when background images are not clipped.) Thus, unless the image is tiled, it may be invisible.

body {
  background: red url("pendant.gif");
  background-repeat: repeat-y;
  background-attachment: fixed;
}

User agents that do not support ‘fixed’ backgrounds (for example due to limitations of the hardware platform) should ignore declarations with the keyword ‘fixed’. For example:

body {
   /* For all UAs: */
   background: white url(paper.png) scroll;
   /* For UAs that do fixed backgrounds: */
   background: white url(ledger.png) fixed;
}
h1 {
   /* For all UAs: */
   background: silver;
   /* For UAs that do fixed backgrounds: */
   background: url(stripe.png) fixed, white url(ledger.png) fixed;
}

See the section “Layering multiple background images” for how ‘background-attachment’ interacts with other comma-separated background properties to form each background image layer.

3.6. Positioning Images: the ‘background-position’ property

If background images have been specified, this property specifies their initial position (after any resizing) within their corresponding background positioning area.

Where

<position> = [
  [ left | center | right | top | bottom | <percentage> | <length> ]
|
  [ left | center | right | <percentage> | <length> ]
  [ top | center | bottom | <percentage> | <length> ]
|
  [ center | [ left | right ] [ <percentage> | <length> ]? ] &&
  [ center | [ top | bottom ] [ <percentage> | <length> ]? ]
]

If only one value is specified, the second value is assumed to be ‘center’. If two values are given, a length or percentage as the first value represents the horizontal position (or offset) and a length or percentage as the second value represents the vertical position (or offset). <percentage> and <length> values here represent an offset of the top left corner of the background image from the top left corner of the background positioning area.

Note that a pair of keywords can be reordered while a combination of keyword and length or percentage cannot. So ‘center left’ is valid while ‘50% left’ is not.

If three or four values are given, then each <percentage> or<length> represents an offset and must be preceded by a keyword, which specifies from which edge the offset is given. For example, ‘background-position: bottom 10px right 20px’ represents a ‘10px’ vertical offset up from the bottom edge and a ‘20px’ horizontal offset leftward from the right edge. If three values are given, the missing offset is assumed to be zero.

Positive values represent an offset inward from the edge of the background positioning area. Negative values represent an offset outward from the edge of the background positioning area.

 background-position: left 10px top 15px;   /* 10px, 15px */
 background-position: left      top     ;   /*  0px,  0px */
 background-position:      10px     15px;   /* 10px, 15px */
 background-position: left          15px;   /*  0px, 15px */
 background-position:      10px top     ;   /* 10px,  0px */
 background-position: left      top 15px;   /*  0px, 15px */
 background-position: left 10px top     ;   /* 10px,  0px */

<percentage>

A percentage for the horizontal offset is relative to (width of background positioning area - width of background image). A percentage for the vertical offset is relative to (height of background positioning area - height of background image), where the size of the image is the size given by ‘background-size’.

For example, with a value pair of ‘0% 0%’, the upper left corner of the image is aligned with the upper left corner of, usually, the box's padding edge. A value pair of ‘100% 100%’ places the lower right corner of the image in the lower right corner of the area. With a value pair of ‘75% 50%’, the point 75% across and 50% down the image is to be placed at the point 75% across and 50% down the area.

Diagram of the meaning of ‘background-position: 75% 50%’.

<length>

A length value gives a fixed length as the offset. For example, with a value pair of ‘2cm 1cm’, the upper left corner of the image is placed 2cm to the right and 1cm below the upper left corner of the background positioning area.

‘top’

Computes to ‘0%’ for the vertical position if one or two values are given, otherwise specifies the top edge as the origin for the next offset.

‘right’

Computes to ‘100%’ for the horizontal position if one or two values are given, otherwise specifies the right edge as the origin for the next offset.

‘bottom’

Computes to ‘100%’ for the vertical position if one or two values are given, otherwise specifies the bottom edge as the origin for the next offset.

‘left’

Computes to ‘0%’ for the horizontal position if one or two values are given, otherwise specifies the left edge as the origin for the next offset.

‘center’

Computes to ‘50%’ (‘left 50%’) for the horizontal position if the horizontal position is not otherwise specified, or ‘50%’ (‘top 50%’) for the vertical position if it is.

body { background: url("banner.jpeg") right top }    /* 100%   0% */
body { background: url("banner.jpeg") top center }   /*  50%   0% */
body { background: url("banner.jpeg") center }       /*  50%  50% */
body { background: url("banner.jpeg") bottom }       /*  50% 100% */

body {
  background-image: url("logo.png");
  background-attachment: fixed;
  background-position: 100% 100%;
  background-repeat: no-repeat;
}

background-position: right 3em bottom 10px

See the section “Layering multiple background images” for how ‘background-position’ interacts with other comma-separated background properties to form each background image layer.

3.7. Painting Area: the ‘background-clip’ property

Determines the background painting area, which determines the area within which the background is painted. The syntax of the property is given with

<box> = border-box | padding-box | content-box

Values have the following meanings:

‘border-box’

The background is painted within (clipped to) the border box.

‘padding-box’

The background is painted within (clipped to) the padding box.

‘content-box’

The background is painted within (clipped to) the content box.

Note that the root element has a different background painting area, and thus the ‘background-clip’ property has no effect when specified on it. See “The backgrounds of special elements.”

Note that the background is always drawn behind the border, if any. See “Elaborate description of Stacking Contexts” in [CSS21].

See the section “Layering multiple background images” for how ‘background-clip’ interacts with other comma-separated background properties to form each background image layer.

3.8. Positioning Area: the ‘background-origin’ property

For elements rendered as a single box, specifies the background positioning area. For elements rendered as multiple boxes (e.g., inline boxes on several lines, boxes on several pages) specifies which boxes ‘box-decoration-break’ operates on to determine the background positioning area(s).

‘padding-box’

The position is relative to the padding box. (For single boxes ‘0 0’ is the upper left corner of the padding edge, ‘100% 100%’ is the lower right corner.)

‘border-box’

The position is relative to the border box.

‘content-box’

The position is relative to the content box.

If the ‘background-attachment’ value for this image is ‘fixed’, then this property has no effect: in this case the background positioning area is the initial containing block [CSS21].

Note that if ‘background-clip’ is ‘padding-box’, ‘background-origin’ is ‘border-box’, ‘background-position’ is ‘top left’ (the initial value), and the element has a non-zero border, then the top and left of the background image will be clipped.

See the section “Layering multiple background images” for how ‘background-origin’ interacts with other comma-separated background properties to form each background image layer.

3.9. Sizing Images: the ‘background-size’ property

Specifies the size of the background images. Where

<bg-size> = [ <length> | <percentage> | auto ]{1,2} | cover | contain

Values have the following meanings:

‘contain’

Scale the image, while preserving its intrinsic aspect ratio (if any), to the largest size such that both its width and its height can fit inside the background positioning area.

‘cover’

Scale the image, while preserving its intrinsic aspect ratio (if any), to the smallest size such that both its width and its height can completely cover the background positioning area.

[ <length> | <percentage> | auto ]{1,2}

The first value gives the width of the corresponding image, the second value its height. If only one value is given the second is assumed to be ‘auto’.

A percentage is relative to the background positioning area.

An ‘auto’ value for one dimension is resolved by using the image's intrinsic ratio and the size of the other dimension, or failing that, using the image's intrinsic size, or failing that, treating it as 100%.

If both values are ‘auto’ then the intrinsic width and/or height of the image should be used, if any, the missing dimension (if any) behaving as ‘auto’ as described above. If the image has neither an intrinsic width nor an intrinsic height, its size is determined as for ‘contain’.

Negative values are not allowed.

div {
    background-image: url(plasma.png);
    background-repeat: no-repeat;
    background-size: 100% 100%;
    background-origin: content-box }

p {
    background-image: url(tubes.png);
    background-size: 50% auto;
    background-origin: border-box }

para {
    background-size: 15px 15px;
    background-image: url(tile.png)}

body {
    background-size: auto;            /* default */
    background-image: url(flower.png) }

p {
    background-image: url(chain.png);
    background-repeat: no-repeat round;
    background-size: 20% 30% }

If ‘background-repeat’ is ‘round’ for one (or both) dimensions, there is a second step. The UA must scale the image in that dimension (or both dimensions) so that it fits a whole number of times in the background positioning area. In the case of the width (height is analogous):

If X ≠ 0 is the width of the image after step one and W is the width of the background positioning area, then the rounded width X' = W / round(W / X) where round() is a function that returns the nearest natural number (integer greater than zero).

If ‘background-repeat’ is ‘round’ for one dimension only and if ‘background-size’ is ‘auto’ for the other dimension, then there is a third step: that other dimension is scaled so that the original aspect ratio is restored.

div {
  background-image: url(image1.png);
  background-repeat: repeat;         /* default */
  background-size: auto }            /* default */

div {
  background-image: url(image2.png);
  background-repeat: repeat;         /* default */
  background-size: 3em }             /* = '3em auto' */

div {
  background-image: url(image3.png);
  background-repeat: round repeat;
  background-size: 3em auto }

div {
  background-image: url(image4.png);
  background-repeat: repeat round;
  background-size: 3em auto }

div {
  background-image: url(image5.png);
  background-repeat: round;
  background-size: auto 4em }

If the background image's width or height resolves to zero, this causes the image not to be displayed. (The effect is the same as if it had been a transparent image.)

See the section “Layering multiple background images” for how ‘background-size’ interacts with other comma-separated background properties to form each background image layer.

3.10. Backgrounds Shorthand: the ‘background’ property

Where

<bg-layer> = <bg-image> || <position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <box>{1,2} 

<final-bg-layer> = <bg-image> || <position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <box>{1,2} || <'background-color'>

Note that a color is permitted in <final-bg-layer>, but not in <bg-layer>.

The ‘background’ property is a shorthand property for setting most background properties at the same place in the style sheet. The number of comma-separated items defines the number of background layers. Given a valid declaration, for each layer the shorthand first sets the corresponding layer of each of ‘background-image’, ‘background-position’, ‘background-size’, ‘background-repeat’, ‘background-origin’, ‘background-clip’ and ‘background-attachment’ to that property's initial value, then assigns any explicit values specified for this layer in the declaration. Finally ‘background-color’ is set to the specified color, if any, else set to its initial value.

If one <box> value is present then it sets both ‘background-origin’ and ‘background-clip’ to that value. If two values are present, then the first sets ‘background-origin’ and the second ‘background-clip’.

body { background: red }
p { background: url("chess.png") 40% / 10em gray
       round fixed border-box; }

body {
    background-color: red;
    background-position: 0% 0%;
    background-size: auto auto;
    background-repeat: repeat repeat;
    background-clip: border-box;
    background-origin: padding-box;
    background-attachment: scroll;
    background-image: none }

p {
    background-color: gray;
    background-position: 40% 50%;
    background-size: 10em 10em;
    background-repeat: round round;
    background-clip: border-box;
    background-origin: border-box;
    background-attachment: fixed;
    background-image: url(chess.png) }

E { background: #CCC url("metal.jpg") top left / 100% auto no-repeat}

div { background: padding-box url(paper.jpg) white center }
div {
    background-color: white;
    background-image: url(paper.jpg);
    background-repeat: repeat;
    background-attachment: scroll;
    background-position: center;
    background-clip: padding-box;
    background-origin: padding-box;
    background-size: auto auto }

background: url(a.png) top left no-repeat,
            url(b.png) center / 100% 100% no-repeat,
            url(c.png) white;

background-image:      url(a.png),  url(b.png),          url(c.png);
background-position:   top left,    center,              top left;
background-repeat:     no-repeat,   no-repeat no-repeat, repeat;
background-clip:       border-box,  border-box,          border-box;
background-origin:     padding-box, padding-box,         padding-box;
background-size:       auto auto,   100% 100%,           auto auto;
background-attachment: scroll,      scroll,              scroll;
background-color:      white;

3.11. Backgrounds of Special Elements

The background of the root element becomes the background of the canvas and its background painting area extends to cover the entire canvas, although any images are sized and positioned relative to the root element as if they were painted for that element alone. (In other words, the background positioning area is determined as for the root element.) If the root's ‘background-color’ value is ‘transparent’, the canvas's background color is UA dependent. The root element does not paint this background again, i.e., the used value of its background is transparent.

For documents whose root element is an HTML HTML element [HTML401] or an XHTML html element [XHTML11]: if the computed value of ‘background-image’ on the root element is ‘none’ and its ‘background-color’ is ‘transparent’, user agents must instead propagate the computed values of the background properties from that element's first HTML BODY or XHTML body child element. The used values of that BODY element's background properties are their initial values, and the propagated values are treated as if they were specified on the root element. It is recommended that authors of HTML documents specify the canvas background for the BODY element rather than the HTML element.

<!DOCTYPE html PUBLIC '-//W3C//DTD HTML 4.0//EN'
  >
<html>
  <head>
    <title>Setting the canvas background</title>
    <style type="text/css">
       body { background: url("http://example.org/marble.png") }
    </style>
  </head>
  <body>
    <p>My background is marble.</p>
  </body>
</html>

The ‘::first-line’ pseudo-element is like an inline-level element for the purposes of the background (see section 5.12.1 of [CSS21]). That means, e.g., that in a left-justified first line, the background does not necessarily extend all the way to the right margin.

4. Borders

The border can either be a predefined style (solid line, double line, dotted line, pseudo-3D border, etc.) or it can be an image. In the former case, various properties define the style (‘border-style’), color (‘border-color’), and thickness (‘border-width’) of the border.

4.1. Line Colors: the ‘border-color’ properties

These properties set the foreground color of the border specified by the border-style properties.

‘Border-color’ is a shorthand for the four ‘border-*-color’ properties. The four values set the top, right, bottom and left border, respectively. A missing left is the same as right, a missing bottom is the same as top, and a missing right is also the same as top.

4.2. Line Patterns: the ‘border-style’ properties

These properties set the style of the border, unless there is a border image

‘Border-style’ is a shorthand for the other four. Its four values set the top, right, bottom and left border respectively. A missing left is the same as right, a missing bottom is the same as top, and a missing right is also the same as top.

Where

<border-style> = none | hidden | dotted | dashed | solid | double | groove | ridge | inset | outset

Values have the following meanings:

‘none’

No border. Color and width are ignored (i.e., the border has width 0, unless the border is an image, see ‘background-image’

‘hidden’

Same as ‘none’, but has different behavior in the border conflict resolution rules for border-collapsed tables [CSS21].

‘dotted’

A series of round dots.

‘dashed’

A series of square-ended dashes.

‘solid’

A single line segment.

‘double’

Two parallel solid lines with some space between them. (The thickness of the lines is not specified, but the sum of the lines and the space must equal ‘border-width’.)

‘groove’

Looks as if it were carved in the canvas. (This is typically achieved by creating a “shadow” from two colors that are slightly lighter and darker than the ‘border-color’.)

‘ridge’

Looks as if it were coming out of the canvas.

‘inset’

Looks as if the content on the inside of the border is sunken into the canvas. Treated as ‘ridge’ in the collapsing border model. [CSS21]

‘outset’

Looks as if the content on the inside of the border is coming out of the canvas. collapsing border model. [CSS21]

Borders are drawn in front of the element's background, but behind the element's content (in case it overlaps).

Note: There is no control over the spacing of the dots and dashes, nor over the length of the dashes. Implementations are encouraged to choose a spacing that makes the corners symmetrical.

Note: This specification does not define how borders of different styles should be joined in the corner. Also note that rounded corners may cause the corners and the contents to overlap, if the padding is less than the radius of the corner.

4.3. Line Thickness: the ‘border-width’ properties

These properties set the thickness of the border. Where

<border-width> = <length> | thin | medium | thick

The lengths corresponding to ‘thin’, ‘medium’ and ‘thick’ are not specified, but the values are constant throughout a document and thin ≤ medium ≤ thick. A UA could, e.g., make the thickness depend on the ‘medium’ font size: one choice might be 1, 3 & 5px when the ‘medium’ font size is 17px or less. Negative <length> values are not allowed.

‘Border-width’ is a shorthand that sets the four ‘border-*-width’ properties. If it has four values, they set top, right, bottom and left in that order. If left is missing, it is the same as right; if bottom is missing, it is the same as top; if right is missing, it is the same as top.

Note that the initial width is ‘medium’, but the initial style is ‘none’ and therefore the used width is 0.

When the used width of the border is 0, we say that the border is absent.

4.4. Border Shorthand Properties

This is a shorthand property for setting the width, style, and color of the top, right, bottom, and left border of a box. Omitted values are set to their initial values.

The ‘border’ property is a shorthand property for setting the same width, color, and style for all four borders of a box. Unlike the shorthand ‘margin’ and ‘padding’ properties, the ‘border’ property cannot set different values on the four borders. To do so, one or more of the other border properties must be used.

The ‘border’ shorthand also resets ‘border-image’ to its initial value. It is therefore recommended that authors use the ‘border’ shorthand, rather than other shorthands or the individual properties, to override any border settings earlier in the cascade. This will ensure that ‘border-image’ has also been reset to allow the new styles to take effect.

The CSS Working Group intends for the ‘border’ shorthand to reset all border properties in future levels of CSS as well. For example, if a ‘border-characters’ property is introduced in the future to allow glyphs as borders, it will also be reset by the ‘border’ shorthand. By using the ‘border’ shorthand to reset borders, authors can be guaranteed a “blank canvas” no matter what properties are introduced in the future.

p { border: solid red }
p {
  border-top: solid red;
  border-right: solid red;
  border-bottom: solid red;
  border-left: solid red;
  border-image: none;
}

Since, to some extent, the properties have overlapping functionality, the order in which the rules are specified is important.

blockquote {
  border-color: red;
  border-left: double;
  color: black
}

5. Rounded Corners

5.1. Curve Radii: the ‘border-radius’ properties

The two length or percentage values of the ‘border-*-radius’ properties define the radii of a quarter ellipse that defines the shape of the corner of the outer border edge (see the diagram below). The first value is the horizontal radius, the second the vertical radius. If the second value is omitted it is copied from the first. If either length is zero, the corner is square, not rounded. Percentages for the horizontal radius refer to the width of the border box, whereas percentages for the vertical radius refer to the height of the border box. Negative values are not allowed.

DIV.standout {
    width: 13em;
    height: 8em;
    border: solid black 1em;
    border-radius: 7.5em 5em }

The ‘border-radius’ shorthand sets all four ‘border-*-radius’ properties. If values are given before and after the slash, then the values before the slash set the horizontal radius and the values after the slash set the vertical radius. If there is no slash, then the values set both radii equally. The four values for each radii are given in the order top-left, top-right, bottom-right, bottom-left. If bottom-left is omitted it is the same as top-right. If bottom-right is omitted it is the same as top-left. If top-right is omitted it is the same as top-left.

border-radius: 4em;

border-top-left-radius:     4em;
border-top-right-radius:    4em;
border-bottom-right-radius: 4em;
border-bottom-left-radius:  4em;

border-radius: 2em 1em 4em / 0.5em 3em;

border-top-left-radius:     2em 0.5em;
border-top-right-radius:    1em 3em;
border-bottom-right-radius: 4em 0.5em;
border-bottom-left-radius:  1em 3em;

5.2. Corner Shaping

The padding edge (inner border) radius is the outer border radius minus the corresponding border thickness. In the case where this results in a negative value, the inner radius is zero. (In such cases its center might not coincide with that of the outer border curve.) Likewise the content edge radius is the padding edge radius minus the corresponding padding, or if that is negative, zero. The border and padding thicknesses in the curved region are thus interpolated from the adjoining sides, and when two adjoining borders are of different thicknesses the corner will show a smooth transition between the thicker and thinner borders.

All border styles (‘solid’, ‘dotted’, ‘inset’, etc.) follow the curve of the border.

Note that if the center of a corner's outer curve is past an opposite padding edge (in the border area of a side opposite the corner), the inner curve will not be a full quarter ellipse.

p { width: 70px; height: 70px; border: solid 30px; border-color: orange orange silver silver; border-top-right-radius: 100%; }

Where the border-radius curve extends into the opposite sides' borders, the arc of the padding edge is less than 90°.

5.3. Corner Clipping

A box's backgrounds, but not its border-image, are clipped to the appropriate curve (as determined by ‘background-clip’). Other effects that clip to the border or padding edge (such as ‘overflow’ other than ‘visible’) also must clip to the curve. The content of replaced elements is always trimmed to the content edge curve.

Also, the area outside the curve of the border edge does not accept pointer events on behalf of the element.

As ‘border-radius’ reduces the interactive area of an element authors should make sure the remaining interactive area conforms to recommended minima for the platforms they target; in particular, conforming to recommended minimum touch target sizes may require larger widths and heights when ‘border-radius’ is used.

DIV {
    background: black;
    color: white;
    border-radius: 1em;
    padding: 1em }

5.4. Color and Style Transitions

Color and style transitions must be contained within the segment of the border that intersects the smallest rectangle that contains both border radii as well as the center of the inner curve (which may be a point representing the corner of the padding edge, if the border radii are smaller than the border-width).

If one of these borders is zero-width, then the other border takes up the entire transitional area. Otherwise, the center of color and style transitions between adjoining borders is a point along the curve that is a continuous monotonic function of the ratio of the border widths. However it is not defined what these transitions look like or what function maps from this ratio to a point on the curve.

5.5. Overlapping Curves

Corner curves must not overlap: When the sum of any two adjacent border radii exceeds the size of the border box, UAs must proportionally reduce the used values of all border radii until none of them overlap. The algorithm for reducing radii is as follows:

Let f = min(L_i/S_i), where i ∈ {top, right, bottom, left}, S_i is the sum of the two corresponding radii of the corners on side i, and L_top = L_bottom = the width of the box, and L_left = L_right = the height of the box. If f < 1, then all corner radii are reduced by multiplying them by f.

Note that this formula ensures that quarter circles remain quarter circles and large radii remain larger than smaller ones, but it may reduce corners that were already small enough, which may make borders of nearby elements that should look the same look different.

If the curve interferes with UI elements such as scrollbars, the UA may further reduce the used value of the affected border radii (and only the affected border radii) as much as necessary, but no more.

box-sizing: border-box;
width: 6em;
height: 2.5em;
border-radius: 0.5em 2em 0.5em 2em

box-sizing: border-box;
width: 6em;
height: 2em;
border-radius: 0.5em 2em 0.5em 2em

5.6. Effect on Tables

The ‘border-radius’ properties do apply to ‘table’ and ‘inline-table’ elements. When ‘border-collapse’ is ‘collapse’, the UA may apply the border-radius properties to ‘table’ and ‘inline-table’ elements, but is not required to. In this case not only must the border radii of adjacent corners not intersect, but the horizontal and vertical radii of a single corner must not extend past the far border edges of the cell at that corner (i.e. a table corner's border-radius does not affect cells not at that corner). If the computed values of the border radii would cause this effect, then the used values of all the border radii of the table must be reduced by the same factor so that the radii neither intersect nor extend past the border edges of their respective corner cells.

The effect of border-radius on internal table elements is undefined in CSS3 Backgrounds and Borders, but may be defined in a future specification. CSS3 UAs should ignore border-radius properties applied to internal table elements when ‘border-collapse’ is ‘collapse’.

6. Border Images

Authors can specify an image to be used in place of the border styles. In this case, the border's design is taken from the sides and corners of an image specified with ‘border-image-source’, whose pieces may be sliced, scaled and stretched in various ways to fit the size of the border image area. The border-image properties do not affect layout: layout of the box, its content, and surrounding content is based on the ‘border-width’ and ‘border-style’ properties only.

DIV {
  border: double orange 1em;
  border-image: url("border.png") 27 round stretch;
 }

    .notebox {
      border: double orange;
      /* must set 'border' shorthand first, otherwise it erases 'border-image' */
      border-image: url("border.png") 30 round;
      /* but other 'border' properties can be set after */
      border-width: thin thick;
    }
    ...
    .sidebar .notebox {
      box-shadow: 0 0 5px gray;
      border-radius: 5px;
      border: none; /* turn off all borders */
      /* 'border' shorthand resets 'border-image' */
    }
  

6.1. Image Source: the ‘border-image-source’ property

Specifies an image to use instead of the border styles given by the ‘border-style’ properties and as an additional background layer for the element. If the value is ‘none’ or if the image cannot be displayed (or the property doesn't apply), the border styles will be used; otherwise the element's borders are invisible and the border image is drawn as described in the sections below.

6.2. Image Slicing: the ‘border-image-slice’ property

This property specifies inward offsets from the top, right, bottom, and left edges of the image, dividing it into nine regions: four corners, four edges and a middle. The middle image part is discarded (treated as fully transparent) unless the ‘fill’ keyword is present. (It is drawn over the background; see Drawing the Border Image.)

When four values are specified, they set the offsets on the top, right, bottom and left sides in that order. If the left is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top.

<percentage>

Percentages are relative to the size of the image: the width of the image for the horizontal offsets, the height for vertical offsets.

<number>

Numbers represent pixels in the image (if the image is a raster image) or vector coordinates (if the image is a vector image).

‘fill’

The ‘fill’ keyword, if present, causes the middle part of the border-image to be preserved. (By default it is discarded, i.e., treated as empty.)

Negative values are not allowed and values bigger than the size of the image are interpreted as ‘100%’.

The regions given by the ‘border-image-slice’ values may overlap. However if the sum of the right and left widths is equal to or greater than the width of the image, the images for the top and bottom edge and the middle part are empty, which has the same effect as if a nonempty transparent image had been specified for those parts. Analogously for the top and bottom values.

If the image must be sized to determine the slices (for example, for SVG images with no intrinsic size), then it is sized as for an auto-sized background, using the border image area as the default object size in place of the background positioning area.

6.3. Drawing Areas: the ‘border-image-width’ property

The border image is drawn inside an area called the border image area. This is an area whose boundaries by default correspond to the border box, see ‘border-image-outset’.

The four values of ‘border-image-width’ specify offsets that are used to divide the border image area into nine parts. They represent inward distances from the the top, right, bottom, and left sides of the area, respectively. If the left width is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top. Values have the following meanings:

<percentage>

Percentages refer to the size of the border image area: the width of the area for horizontal offsets, the height for vertical offsets.

<number>

Numbers represent multiples of the corresponding computed border-width.

‘auto’

If ‘auto’ is specified then the border image width is the intrinsic width or height (whichever is applicable) of the corresponding image slice (see ‘border-image-slice’). If the image does not have the required intrinsic dimension then the corresponding border-width is used instead.

Negative values are not allowed for any ‘border-image-width’ values.

If two opposite ‘border-image-width’ offsets are large enough that they overlap, then the used values of all ‘border-image-width’ offsets are proportionally reduced until they no longer overlap. In mathematical notation: Given L_width as the width of the border image area, L_height as its height, and W_side as the border image width offset for the side side, let f = min(L_width/(W_left+W_right), L_height/(W_top+W_bottom)). If f < 1, then all W are reduced by multiplying them by f.

6.4. Edge Overhang: the ‘border-image-outset’ property

The values specify the amount by which the border image area extends beyond the border box. If it has four values, they set the outsets on the top, right, bottom and left sides in that order. If the left is missing, it is the same as the right; if the bottom is missing, it is the same as the top; if the right is missing, it is the same as the top.

As with ‘border-image-width’, a <number> represents a multiple of the corresponding border-width. Negative values are not allowed for any of the ‘border-image-outset’ values.

Portions of the border-image that are rendered outside the border box do not trigger scrolling. Also such portions are invisible to mouse events and do not capture such events on behalf of the element.

Note that, even though they never cause a scrolling mechanism, outset images may still be clipped by an ancestor or by the viewport.

6.5. Image Tiling: the ‘border-image-repeat’ property

This property specifies how the images for the sides and the middle part of the border image are scaled and tiled. The first keyword applies to the horizontal sides, the second to the vertical ones; see Drawing the Border Image. If the second keyword is absent, it is assumed to be the same as the first. Values have the following meanings:

‘stretch’

The image is stretched to fill the area.

‘repeat’

The image is tiled (repeated) to fill the area.

‘round’

The image is tiled (repeated) to fill the area. If it does not fill the area with a whole number of tiles, the image is rescaled so that it does.

‘space’

The image is tiled (repeated) to fill the area. If it does not fill the area with a whole number of tiles, the extra space is distributed around the tiles.

The exact process for scaling and tiling the border-image parts is given in the section below.

6.6. Drawing the Border Image

After the border-image given by ‘border-image-source’ is sliced by the ‘border-image-slice’ values, the resulting nine images are scaled, positioned, and tiled into their corresponding border image regions in four steps:

1. Scale to ‘border-image-width’.

   • The two images for the top and bottom edges are made as tall as the top and bottom border image area parts, respectively, and their width is scaled proportionally.

   • The images for the left and right edge are made as wide as the left and right border image area parts, respectively, and their height is scaled proportionally.

   • The corner images are scaled to be as wide and as tall as the two border-image edges they are part of.

   • The middle image's width is scaled by the same factor as the top image unless that factor is zero or infinity, in which case the scaling factor of the bottom is substituted, and failing that, the width is not scaled. The height of the middle image is scaled by the same factor as the left image unless that factor is zero or infinity, in which case the scaling factor of the right image is substituted, and failing that, the height is not scaled.

2. Scale to ‘border-image-repeat’.

   • If the first keyword is ‘stretch’, the top, middle and bottom images are further scaled to be as wide as the middle part of the border image area. The height is not changed any further.

   • If the first keyword is ‘round’, the top, middle and bottom images are resized in width, so that exactly a whole number of them fit in the middle part of the border-image area, exactly as for ‘round’ in the ‘background-repeat’ property.

   • If the first keyword is ‘repeat’ or ‘space’, the top, middle, and bottom images are not changed any further.

   • The effects of ‘stretch’, ‘round’, ‘repeat’, and ‘space’ for the second keyword are analogous, acting on the height of the left, middle and right images.

3. Position the first tile.

   • If the first keyword is ‘repeat’, the top, middle, and bottom images are centered horizontally in their respective areas. Otherwise the images are placed at the left edge of their respective parts of the border-image area.

   • If the second keyword is ‘repeat’, the left, middle, and right images are centered vertically in their respective areas. Otherwise the images are placed at the top edge of their respective parts of the border-image area.

4. Tile and draw.

   • The images are then tiled to fill their respective areas. All images are drawn at the same stacking level as normal borders: immediately in front of the background layers.

   • In the case of ‘space’, any partial tiles are discarded and the extra space distributed before, after, and between the tiles. (I.e. the gap before the first tile, the gap after the last tile, and the gaps between tiles are equalized.) This can result in empty border-image side areas.

6.7. Border Image Shorthand: the ‘border-image’ property

This is a shorthand property for setting ‘border-image-source’, ‘border-image-slice’, ‘border-image-width’, ‘border-image-outset’ and ‘border-image-repeat’. Omitted values are set to their initial values.

6.8. Effect on Tables

The ‘border-image’ properties apply to the border of tables and inline tables that have ‘border-collapse’ set to ‘collapse’. However, this specification does not define how such an image border is rendered. In particular, it does not define how the image border interacts with the borders of cells, rows and row groups at the edges of the table (see border conflict resolution in [CSS21]).

It is expected that a future specification will define the rendering. It is recommended that UAs do not apply border images to tables with collapsed borders until then.

7. Miscellaneous Effects

7.1. Drawing at Breaks: the ‘box-decoration-break’ property

When a box is broken at a page break, column break, or, for inline elements, at a line break, the ‘box-decoration-break’ property specifies whether box fragments are treated as broken fragments of one continuous box, or whether each fragment is individually wrapped with the border and padding. For backgrounds it defines how the background positioning area is derived from these multiple fragments and how the element's background is drawn within them. Values have the following meanings:

‘clone’

Each box fragment is independently wrapped with the border and padding. The ‘border-radius’ and ‘border-image’ and ‘box-shadow’, if any, are applied to each fragment independently. The background is drawn independently in each fragment of the element. A no-repeat background image will thus be rendered once in each fragment of the element.

‘slice’

No border and no padding are inserted at a break. No box-shadow is drawn at a broken edge; ‘border-radius’ does not apply to its corners; and the ‘border-image’ is rendered for the whole box as if it were unbroken. The effect is as though the element were rendered with no breaks present, and then sliced by the breaks afterward.

Backgrounds are drawn as if, after the element has been laid out (including any justification, bidi reordering, page breaks, etc.), all the element's box fragments were taken and put one after the other in visual order. The background is applied to the bounding box of this composite box and then the fragments are put back, with each with its share of the background.

For boxes broken across lines, first fragments on the same line are connected in visual order. Then fragments on subsequent lines are ordered according to the element's inline progression direction and aligned on the baseline. For example in a left-to-right containing block (‘direction’ is ‘ltr’), the first fragment is the leftmost fragment on the first line and fragments from subsequent lines are put to the right of it. In a right-to-left containing block, the first fragment is the rightmost on the first line and subsequent fragments are put to the left of it.

For boxes broken across columns, the columns are treated as one continuous element, as if the column boxes were glued together in the block progression direction of the multi-column element. For boxes broken across pages, the page content areas are glued together in the block progression direction of the root element. In these cases, if the box fragments have different widths (heights, if the root element / multi-column element is in a vertical writing mode), then each piece draws its background assuming that the whole element has the same width (height) as this piece. This ensures that right-aligned images stay aligned to the right edge, left-aligned images stay aligned to the left edge, and centered images stay centered.

For inline elements, which side of a fragment is the broken edge is determined by the parent element's inline progression direction. For example, if an inline element whose parent has ‘direction: rtl’ breaks across two lines, the left edge of the fragment on the first line will be the broken edge. (Note in particular that neither the element's own ‘direction’ nor its containing block's ‘direction’ is used.) See [CSS3-WRITING-MODES].

UAs may also apply ‘box-decoration-break’ to control rendering at bidi-imposed breaks, i.e. when bidi reordering causes an inline to split into non-contiguous fragments. Otherwise such breaks are always handled as ‘slice’.

7.2. Drop Shadows: the ‘box-shadow’ property

The ‘box-shadow’ property attaches one or more drop-shadows to the box. The property is a comma-separated list of shadows, each specified by 2-4 length values, an optional color, and an optional ‘inset’ keyword. Omitted lengths are 0; omitted colors are a UA-chosen color.

Where

<shadow> = inset? && [ <length>{2,4} && <color>? ]

The components of each <shadow> are interpreted as follows:

• The first length is the horizontal offset of the shadow. A positive value draws a shadow that is offset to the right of the box, a negative length to the left.
• The second length is the vertical offset. A positive value offsets the shadow down, a negative one up.
• The third length is a blur radius. Negative values are not allowed. If the blur value is zero, the shadow's edge is sharp. Otherwise, the larger the value, the more the shadow's edge is blurred. See below.
• The fourth length is a spread distance. Positive values cause the shadow shape to expand in all directions by the specified radius. Negative values cause the shadow shape to contract. See below. Note that for inner shadows, expanding the shadow (creating more shadow area) means contracting the shadow's perimeter shape.
• The color is the color of the shadow. If the color is absent, the used color is taken from the ‘color’ property.
• The ‘inset’ keyword, if present, changes the drop shadow from an outer shadow (one that shadows the box onto the canvas, as if it were lifted above the canvas) to an inner shadow (one that shadows the canvas onto the box, as if the box were cut out of the canvas and shifted behind it).

An outer box-shadow casts a shadow as if the border-box of the element were opaque. The shadow is drawn outside the border edge only: it is clipped inside the border-box of the element.

An inner box-shadow casts a shadow as if everything outside the padding edge were opaque. The inner shadow is drawn inside the padding edge only: it is clipped outside the padding box of the element.

If the box has a nonzero ‘border-radius’, the shadow shape is rounded in the same way. The ‘border-image’ does not affect the shape of the box-shadow.

If a spread distance is defined, the shadow is expanded outward or contracted inward by an operation equivalent to applying the absolute value of the spread value to a blur operation as defined below and thresholding the result such that for a positive spread distance all non-transparent pixels are given the full shadow color and for a negative spread distance all non-opaque pixels are made transparent. The UA may approximate this operation by taking an outward outset of the specified amount normal to the original shadow perimeter. Alternatively the UA may approximate the transformed shadow perimeter shape by outsetting (insetting, for inner shadows) the shadow's straight edges by the spread distance and increasing (decreasing, for inner shadows) and flooring at zero the corner radii by the same amount. (The UA may even combine these methods, using one method for outer shadows and another for inner ones.) For corners with a zero border-radius, however, the corner must remain sharp—the operation is equivalent to scaling the shadow shape. In any case, the effective width and height of the shadow shape is floored at zero. (A zero-sized shadow shape would cause an outer shadow to disappear, and an inner shadow to cover the entire padding-box.)

A non-zero blur distance indicates that the resulting shadow should be blurred, such as by a Gaussian filter. The exact algorithm is not defined; however the resulting shadow must approximate (with each pixel being within 5% of its expected value) the image that would be generated by applying to the shadow a Gaussian blur with a standard deviation equal to half the blur radius

Note this means for a long, straight shadow edge, the blur radius will create a visibly apparent color transition approximately the twice length of the blur radius that is perpendicular to and centered on the shadow's edge, and that ranges from the full shadow color at the endpoint inside the shadow to fully transparent at the endpoint outside it.

Width: 100px; Height: 100px;
Border: 12px solid blue; Background-color: orange;
Border-top-left-radius: 60px 90px;
Border-bottom-right-radius: 60px 90px;
Box-shadow: 64px 64px 12px 40px rgba(0,0,0,0.4),
           12px 12px 0px 8px rgba(0,0,0,0.4) inset;

The shadow effects are applied front-to-back: the first shadow is on top and the others are layered behind. Shadows do not influence layout and may overlap other boxes or their shadows. In terms of stacking contexts and the painting order, the outer shadows of an element are drawn immediately below the background of that element, and the inner shadows of an element are drawn immediately above the background of that element (below the borders and border image, if any).

If an element has multiple boxes, all of them get drop shadows, but shadows are only drawn where borders would also be drawn; see ‘box-decoration-break’.

Shadows do not trigger scrolling or increase the size of the scrollable area.

Below are some examples of an orange box with a blue border being given a drop shadow.

border:5px solid blue; background-color:orange; width: 144px; height: 144px;	border-radius: 20px;	border-radius: 0;
box-shadow: rgba(0,0,0,0.4) 10px 10px;
box-shadow: rgba(0,0,0,0.4) 10px 10px inset
box-shadow: rgba(0,0,0,0.4) 10px 10px 0 10px /* spread */
box-shadow: rgba(0,0,0,0.4) 10px 10px 0 10px /* spread */ inset

Outer shadows have no effect on internal table elements in the collapsing border model. If a shadow is defined for single border edge in the collapsing border model that has multiple border thicknesses (e.g. an outer shadow on a table where one row has thicker borders than the others, or an inner shadow on a rowspanning table cell that adjoins cells with different border thicknesses), the exact position and rendering of its shadows are undefined.

8. Definitions

8.1. Glossary

The following terms and abbreviations are used in this module.

UA

User Agent

A program that reads and/or writes CSS style sheets on behalf of a user in either or both of these categories: programs whose purpose is to render documents (e.g., browsers) and programs whose purpose is to create style sheets (e.g., editors). A UA may fall into both categories. (There are other programs that read or write style sheets, but this module gives no rules for them.)

document

A tree-structured document with elements and attributes, such as an SGML or XML document [XML11].

style sheet

A CSS style sheet.

8.2. Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification. All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Conformance to CSS Backgrounds and Borders Level 3 is defined for three classes:

style sheet

A CSS style sheet.

renderer

A UA that interprets the semantics of a style sheet and renders documents that use them.

authoring tool

A UA that writes a style sheet.

A style sheet is conformant to CSS Backgrounds and Borders Level 3 if all of its declarations that use properties defined in this module have values that are valid according to the generic CSS grammar and the individual grammars of each property as given in this module.

A renderer is conformant to CSS Backgrounds and Borders Level 3 if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the properties defined by CSS Backgrounds and Borders Level 3 by parsing them correctly and rendering the document accordingly. However the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to CSS Backgrounds and Borders Level 3 if it writes syntactically correct style sheets, according to the generic CSS grammar and the individual grammars of each property in this module.

8.3. Levels

This section is informative. CSS has different levels of features, each a subset of the other. (See [CSS-2010] for a full explanation.) The lists below describe which features from this specification are in each level.

8.3.1. Level 1

• ‘background-color’
• ‘background-image’ only one image (no layers)
• ‘background-repeat’: only ‘repeat’ | ‘repeat-x’ | ‘repeat-y’ | ‘no-repeat’
• ‘background-attachment’: only ‘scroll’ | ‘fixed’
• ‘background-position’: only one or two values allowed
• ‘background’ shorthand: only color, image, repeat, attachment and position
• ‘border-color’ properties
• ‘border-style’ properties
• ‘border-width’ properties
• ‘border-top’, ‘border-bottom’, ‘border-right’, ‘border-left’, and ‘border’ shorthands

8.3.2. Level 2

• ‘background-color’
• ‘background-image’: only one image (no layers)
• ‘background-repeat’: only ‘repeat’ | ‘repeat-x’ | ‘repeat-y’ | ‘no-repeat’
• ‘background-attachment’: only ‘scroll’ | ‘fixed’
• ‘background-position’: only one or two values allowed
• ‘background’: only color, image, repeat, attachment and position
• ‘border-color’ properties
• ‘border-style’ properties
• ‘border-width’ properties
• ‘border-top’, ‘border-bottom’, ‘border-right’, ‘border-left’, and ‘border’ shorthands

8.3.3. Level 3

• All features described in the CSS Backgrounds and Borders Module Level 3

8.4. CR Exit Criteria

As described in the W3C process document, a Candidate Recommendation (CR) is a specification that W3C recommends for use on the Web. The next stage is “Recommendation,” when the specification is sufficiently implemented.

For this specification to be proposed as a W3C Recommendation, the following conditions shall be met. There must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

A minimum of three months of the CR period must have elapsed. This is to ensure that enough time is given for any remaining major errors to be caught.

Features will be dropped if two or more interoperable implementations are not found by the end of the CR period.

Features may/will also be dropped if adequate/sufficient (by judgment of CSS WG) tests have not been produced for those feature(s) by the end of the CR period.

9. Changes

9.1. Changes since the 17 April 2012 Candidate Recommendation

The following (non-editorial) changes were made to this specification since the 17 April 2012 Candidate Recommendation:

• Fix error in computed value of ‘background-position’: the computed value is a list of positions.
• Add a note pointing out that when ‘border-radius’ reduces the interactive area, the width/height of the box might need to be increased.

9.2. Changes since the 14 February 2012 “Last Call” Working Draft

The following (non-editorial) changes were made to this specification since the 14 February 2012 “Last Call” Working Draft:

These changes were in response to comments received during the review period. For details, see the full Disposition of Comments.

• Section 2.1: Defined which properties from this module apply to the ‘::first-line’ and ‘::first-letter’ pseudo-elements.
• Section 3.4: Fixed the incorrect definition of the computed value of ‘background-repeat’. The value is always a pair of keywords, never a single keyword.
• Section 3.10: Added the missing ‘background-image’ to the list of properties that the ‘background’ property reset.
• Section 4.3: Added that negative lengths are not allowed on ‘border-width’.
• Section 5.1: Added that negative lengths are not allowed on ‘border-radius’.
• Section 6.8: Added a section about the effect of ‘border-image’ on tables with collapsed borders and added that the rendering will be defined later.

9.3. Changes Since the 15 February 2011 Candidate Recommendation

The following changes were made to this specification since the 15 February 2011 Candidate Recommendation:

• Section 3.9: Defined what happens if ‘background-position’ has two ‘auto’ values and the image is missing an intrinsic dimension.

  If both values are ‘auto’ then the intrinsic width and/or height of the image should be used, if any, the missing dimension (if any) behaving as ‘auto’ as described above.

• Section 5.4: Made center of color and style transitions undefined (within certain limits) on corner joins, since previous definition was wrong.

  The center of color and style transitions between adjoining borders is at the point on the curve that is at an angle that is proportional to the ratio of the border widths. For example, if the top and right border widths are equal, that point is at a 45° angle from the horizontal, and if the top is twice the width of the right the point is at a 30° angle from the horizontal. The line demarcating this transition is drawn between the point at that angle on the outer arc and the point at that angle on the inner arc.

  If one of these borders is zero-width, then the other border takes up the entire transitional area. Otherwise, the center of color and style transitions between adjoining borders must be proportional to the ratio of the border widths such that a function of its location is continuous with respect to this ratio. However it is not defined what these transitions look like or how “proportional” maps to a point on the curve.

• Section 6.2: Defined slicing of border images that must be sized first to determine slice positions. Added:

  If the image must be sized to determine the slices (for example, for SVG images with no intrinsic size), then it is sized as for an auto-sized background, using the border image area as the default object size in place of the background positioning area.

• Section 7.1 Optionally added bidi-imposed breaks to the types of breaks affected by ‘box-decoration-break’. Added:

  UAs may also apply ‘box-decoration-break’ to control rendering at bidi-imposed breaks, i.e. when bidi reordering causes an inline to split into non-contiguous fragments. Otherwise such breaks are always handled as ‘slice’.

• Section 7.2: Defined the default color of ‘box-shadow’.

  The color is the color of the shadow. If the color is absent, the used color is taken from the ‘color’ property.

• Section 3.6: Clarified computed value of ‘background-position’.

  If one or two values are specified, for a <length> the absolute length, otherwise a percentage. If three or four values are specified, two pairs of a keyword plus a length or percentage. Two keywords representing the origin and two offsets from that origin, each given as an absolute length (if given a <length>), otherwise as a percentage.

  Changed Equivalent to Computes in definition of keywords.

• Section 2.3 (and all property definitions): Added a section about expected an expected animations module. Added "Animatable" line to property definition tables.
• Section 3.6: Renamed <bg-position> production to <position> for easier re-use in other specifications and recast the grammar to be more verbose but easier to understand.

  bg-position<position> = [
    [ left | center | right | top | bottom | <percentage> | <length> ]
  |
    [ left | center | right | <percentage> | <length> ]
    [ top | center | bottom | <percentage> | <length> ]?
  |
    [ center | [ left | right ] [ <percentage> | <length> ]? ] &&
    [ center | [ top | bottom ] [ <percentage> | <length> ]? ]
  ]

• Section 6.5: Added ‘space’ keyword to ‘border-image-repeat’ property value table: it was included in the list of allowable values, but not in the syntax definition.

  [ stretch | repeat | round | space ]{1,2}

• Section 5.5: Corrected math error in border-radius example.

  The height (2.5em) is enough for the specified radii (0.5em plus 2.5em 2.0em).

• Section 4.3: Marked Percentages field as N/A for ‘border-width’, since they are not included in the property.

9.4. Changes Since the 17 December 2009 Candidate Recommendation

The following changes were made to this specification since the 17 December 2009 Candidate Recommendation:

• Addition of ‘content-box’ value of ‘background-clip’.
• Change to the ‘background’ shorthand syntax for ‘background-clip’ and ‘background-origin’.
• Removal of recommendation to use gradients for color transitions when ‘border-radius’ produces a curve.
• (Re)Addition of ‘box-shadow’ property.
• Various clarifications.

10. Acknowledgments

Tapas Roy was editor of the Border Module, before it was merged with the Background Module.

Thanks to Ben Stucki for defining what happens with rounded corners if the two adjoining borders are of unequal thickness or one of them is zero; to Arjan Eising and Anne van Kesteren for the ‘border-radius’ syntax; to Zack Weinberg for the corner transition regions diagram.

A set of properties for border images was initially proposed by fantasai. The current simplification (one image cut into nine parts) is due to Ian Hickson. (Though the original idea seems to originate with some anonymous Microsoft engineers.)

Finally, special thanks go to Brad Kemper for his feedback and suggestions for many of the features in the draft, for drawing all the box-shadow examples, and for proposing some radical changes to the ‘border-image’ property that solved a number of problems with the earlier definition.

11. References

Normative References

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[HTML401]

Dave Raggett; Arnaud Le Hors; Ian Jacobs. HTML 4.01 Specification. 24 December 1999. W3C Recommendation. URL: http://www.w3.org/TR/1999/REC-html401-19991224

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

[XHTML11]

Murray Altheim; Shane McCarron. XHTML™ 1.1 - Module-based XHTML. 31 May 2001. W3C Recommendation. URL: http://www.w3.org/TR/2001/REC-xhtml11-20010531

Other References

[CSS-2010]

Elika J. Etemad. Cascading Style Sheets (CSS) Snapshot 2010. 12 May 2011. W3C Working Group Note. URL: http://www.w3.org/TR/2011/NOTE-css-2010-20110512/

[CSS1]

Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411

[CSS3-IMAGES]

Elika J. Etemad; Tab Atkins Jr. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-images-20120417/

[CSS3-TRANSITIONS]

Dean Jackson; et al. CSS Transitions. 3 April 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-transitions-20120403/

[CSS3-WRITING-MODES]

Elika J. Etemad; Koji Ishii. CSS Writing Modes Module Level 3. 1 May 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2012/WD-css3-writing-modes-20120501/

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[WCAG20]

Ben Caldwell; et al. Web Content Accessibility Guidelines (WCAG) 2.0. 11 December 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-WCAG20-20081211/

[XML11]

Eve Maler; et al. Extensible Markup Language (XML) 1.1 (Second Edition). 16 August 2006. W3C Recommendation. URL: http://www.w3.org/TR/2006/REC-xml11-20060816

Property Index

Index

• <attachment>, 3.5.
• authoring tool, 8.2.
• ‘auto’
  • ‘background-size’ value, 3.9.
  • ‘border-image-width’ value, 6.3.
• background, 3.10.
• background-attachment, 3.5.
• background-clip, 3.7.
• background-color, 3.2.
• background-image, 3.3.
• background-origin, 3.8.
• background painting area, 3.7., 3.4., 3.4., 3.4., 3.5., 3.11.
• background-position, 3.6.
• background positioning area, 3.8., 3.4., 3.9., 3.4., 3.6., 7.1.
• background-repeat, 3.4.
• background-size, 3.9.
• <bg-image>, 3.3.
• <bg-layer>, 3.10.
• <bg-size>, 3.9.
• border, 4.4.
• border-bottom, 4.4.
• border-bottom-color, 4.1.
• border-bottom-left-radius, 5.1.
• border-bottom-right-radius, 5.1.
• border-bottom-style, 4.2.
• border-bottom-width, 4.3.
• ‘border-box’
  • ‘background-clip’ value, 3.7.
  • ‘background-origin’ value, 3.8.
• border-color, 4.1.
• border-image, 6.7.
• border image area, 6.4.
• border-image-outset, 6.4.
• border-image-repeat, 6.5.
• border-image-slice, 6.2.
• border-image-source, 6.1.
• border-image-width, 6.3.
• border-left, 4.4.
• border-left-color, 4.1.
• border-left-style, 4.2.
• border-left-width, 4.3.
• border-radius, 5.1.
• border-right, 4.4.
• border-right-color, 4.1.
• border-right-style, 4.2.
• border-right-width, 4.3.
• <border-style>, 4.2.
• border-style, 4.2.
• border-top, 4.4.
• border-top-color, 4.1.
• border-top-left-radius, 5.1.
• border-top-right-radius, 5.1.
• border-top-style, 4.2.
• border-top-width, 4.3.
• <border-width>, 4.3.
• border-width, 4.3.
• ‘bottom’
  • ‘background-position’ value, 3.6.
• <box>, 3.7.
• box-decoration-break, 7.1.
• box-shadow, 7.2.
• ‘center’
  • ‘background-position’ value, 3.6.
• ‘clone’, 7.1.
• color
  • background, 3.2.
• ‘contain’, 3.9.
• ‘content-box’, 3.8.
  • ‘background-clip’ value, 3.7.
• ‘cover’, 3.9.
• ‘dashed’, 4.2.
• document, 8.1.
• documents, 8.1.
• ‘dotted’, 4.2.
• ‘double’, 4.2.
• ‘fill’, 6.2.
• <final-bg-layer>, 3.10.
• fixed, 3.5.
• ‘fixed’, 3.5.
• ‘groove’, 4.2.
• ‘hidden’, 4.2.
• <image>, 2.2.
• ‘inset’, 4.2.
• layers, 3.1.
• ‘left’
  • ‘background-position’ value, 3.6.
• local, 3.5.
• ‘local’, 3.5.
• ‘none’
  • ‘background-image’ value, 3.3.
  • ‘border-style’ value, 4.2.
• ‘no-repeat’
  • ‘background-repeat’ value, 3.4.
• ‘outset’, 4.2.
• ‘padding-box’
  • ‘background-clip’ value, 3.7.
  • ‘background-origin’ value, 3.8.
• <position>, 3.6.
• renderer, 8.2.
• ‘repeat’
  • ‘background-repeat’ value, 3.4.
  • ‘border-image-repeat’ value, 6.5.
• <repeat-style>, 3.4.
• ‘repeat-x’, 3.4.
• ‘repeat-y’, 3.4.
• ‘ridge’, 4.2.
• ‘right’
  • ‘background-position’ value, 3.6.
• ‘round’
  • ‘background-repeat’ value, 3.4.
  • ‘border-image-repeat’ value, 6.5.
• scroll, 3.5.
• ‘scroll’, 3.5.
• <shadow>, 7.2.
• ‘slice’, 7.1.
• ‘solid’, 4.2.
• ‘space’
  • ‘background-repeat’ value, 3.4.
  • ‘border-image-repeat’ value, 6.5.
• ‘stretch’, 6.5.
• style sheet, 8.1.
  • as conformance class, 8.2.
• ‘top’
  • ‘background-position’ value, 3.6.
• UA, 8.1.
• User Agent, 8.1.
• viewport, 3.5.