CSS Pseudo-Elements Module Level 4

1. Introduction

This section is informative.

Pseudo-elements represent abstract elements of the document beyond those elements explicitly created by the document language. Since they are not restricted to fitting into the document tree, they can be used to select and style portions of the document that do not necessarily map to the document’s tree structure. For instance, the ::first-line pseudo-element can select content on the first formatted line of an element after text wrapping, allowing just that line to be styled differently from the rest of the paragraph.

Each pseudo-element is associated with an originating element and has syntax of the form ::name-of-pseudo. This module defines the pseudo-elements that exist in CSS and how they can be styled. For more information on pseudo-elements in general, and on their syntax and interaction with other selectors, see [SELECTORS4].

2. Typographic Pseudo-elements

2.1. The ::first-line pseudo-element

The ::first-line pseudo-element describes the contents of the first formatted line of its originating element.

p::first-line { text-transform: uppercase }

Note: Note that the length of the first line depends on a number of factors, including the width of the page, the font size, etc.

<P>This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.

<P><P::first-line> This is a somewhat long HTML
paragraph that </P::first-line> will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

If a pseudo-element breaks up a real element, the desired effect can often be described by a fictional tag sequence that closes and then re-opens the element.

<P><SPAN class="test"> This is a somewhat long HTML
paragraph that will be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

<P><P::first-line><SPAN class="test"> This is a somewhat long HTML
paragraph that will </SPAN></P::first-line><SPAN class="test"> be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

2.1.1. Finding the First Formatted Line

In CSS, the ::first-line pseudo-element can only have an effect when attached to a block container. The first formatted line of an element must occur inside a block-level descendant in the same flow (i.e., a block-level descendant that is not out-of-flow due to floating or positioning).

The first line of a table-cell or inline-block cannot be the first formatted line of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first formatted line of the DIV is not the line "Hello".

Note: Note that the first line of the p in this fragment: <p><br>First... doesn’t contain any letters (assuming the default style for br). The word "First" is not on the first formatted line.

A user agent must act as if the fictional start tags of a ::first-line pseudo-element were nested just inside the innermost enclosing block-level element.

<DIV>
  <P>First paragraph</P>
  <P>Second paragraph</P>
</DIV>

<DIV>
  <P><DIV::first-line><P::first-line>First paragraph</P::first-line></DIV::first-line></P>
  <P><P::first-line>Second paragraph</P::first-line></P>
</DIV>

2.1.2. Styling the First Line Pseudo-element

The ::first-line pseudo-element’s generated box behaves similar to that of an inline-level element, but with certain restrictions. The following CSS properties apply to a ::first-line pseudo-element:

• all font properties (see [CSS3-FONTS])
• the color and opacity properties (see [CSS3COLOR])
• all background properties (see [CSS3BG])
• any typesetting properties that apply to inline elements (see [CSS3TEXT])
• all text decoration properties (see [CSS3-TEXT-DECOR])
• any inline layout properties that apply to inline elements (see [CSS-INLINE-3])
• any other properties defined to apply to ::first-line by their respective specifications

2.1.3. Inheritance and the ::first-line Pseudo-element

During CSS inheritance, the portion of a child element that occurs on the first line only inherits properties applicable to the ::first-line pseudo-element from the ::first-line pseudo-element. For all other properties inheritance is from the non-pseudo-element parent of the first line pseudo element. (The portion of a child element that does not occur on the first line always inherits from the parent of that child.)

2.2. The ::first-letter pseudo-element

The ::first-letter pseudo-element represents the first typographic letter unit [CSS3TEXT] on the first formatted line of its originating element, if it is not preceded by any other content (such as images or inline tables) on its line. The ::first-letter pseudo-element can be used to create “initial caps” and “drop caps”, which are common typographic effects.

h2 + p::first-letter { initial-letter: 2; }

Punctuation (i.e, characters that belong to the Punctuation (P*) Unicode general category [UAX44]) that precedes or follows the first typographic letter unit must also be included in the ::first-letter pseudo-element.

As explained in [CSS3TEXT], a typographic letter unit can include more than one Unicode codepoint. For example, combining characters must be kept with their base character. Also, languages may have additional rules about how to treat certain letter combinations. In Dutch, for example, if the letter combination "ij" appears at the beginning of an element, both letters should be considered within the ::first-letter pseudo-element. [UAX29] The UA should tailor its definition of typographic letter unit to reflect the first-letter traditions of the ::first-letter pseudo-element’s containing block’s content language.

Note: Note that the first typographic letter unit may in fact be a digit, e.g., the “6” in “67 million dollars is a lot of money.”

If the characters that would form the ::first-letter are not in the same element, such as ‘T in <p>‘<em>T..., the user agent may create a ::first-letter pseudo-element from one of the elements, both elements, or simply not create a pseudo-element. Additionally, if the first letter(s) of the block are not at the start of the line (for example due to bidirectional reordering), then the user agent need not create the pseudo-element(s).

The ::first-letter pseudo-element is contained within any ::first-line pseudo-elements, and thus inherits from ::first-line.

2.2.1. Finding the First Letter

The first letter must occur on the first formatted line. For example, in this HTML fragment: <p><br>First... the first line doesn’t contain any letters and ::first-letter doesn’t match anything. In particular, it does not match the “F” of “First”.

In CSS, the ::first-letter pseudo-element only applies to block containers. A future version of this specification may allow this pseudo-element to apply to more display types. The ::first-letter pseudo-element can be used with all such elements that contain text, or that have a descendant in the same flow that contains text. A user agent should act as if the fictional start tag of the ::first-letter pseudo-element is just before the first text of the element, even if that first text is in a descendant.

<div>
<p>The first text.

<div>
<p><div::first-letter><p::first-letter>T</...></...>he first text.

In CSS the first letter of a table-cell or inline-block cannot be the first letter of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first letter of the DIV is not the letter "H". In fact, the DIV doesn’t have a first letter. If an element is a list item (display: list-item), the ::first-letter applies to the first letter in the principal box after the marker. User-Agents may ignore ::first-letter on list items with list-style-position: inside. If an element has ::before or ::after content, the ::first-letter applies to the first letter of the element including that content.

2.2.2. Styling the ::first-letter Pseudo-element

In CSS a ::first-letter pseudo-element is similar to an inline-level element. The following properties that apply to ::first-letter pseudo-elements:

• all font properties (see [CSS3-FONTS])
• the color and opacity properties (see [CSS3COLOR])
• all background properties (see [CSS3BG])
• any typesetting properties that apply to inline elements (see [CSS3TEXT])
• all text decoration properties (see [CSS3-TEXT-DECOR])
• any inline layout properties that apply to inline elements (see [CSS-INLINE-3])
• margin and padding properties (see [CSS21])
• border properties and box-shadow (see [CSS3BG])
• any other properties defined to apply to ::first-letter by their respective specifications

User agents may apply other properties as well.

Note: In previous levels of CSS, User Agents were allowed to choose a line height, width and height based on the shape of the letter, approximate font sizes, or to take the glyph outline into account when formatting. This possibility has been intentionally removed, as it proved to be a poor solution for the intended use case (Drop Caps), yet caused interoperability problems.

Example: This CSS and HTML example shows a possible rendering of an initial cap. Note that the fictional start tag of the first letter is inside the span, and thus the font weight of the first letter is normal, not bold as the span:

p { line-height: 1.1 }
p::first-letter { font-size: 3em; font-weight: normal }
span { font-weight: bold }
...
<p><span>Het hemelsche</span> gerecht heeft zich ten lange lesten<br>
Erbarremt over my en mijn benaeuwde vesten<br>
En arme burgery, en op mijn volcx gebed<br>
En dagelix geschrey de bange stad ontzet.

The following CSS will make a drop cap initial letter span about two lines:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
 <HEAD>
  <TITLE>Drop cap initial letter</TITLE>
  <STYLE type="text/css">
   P               { font-size: 12pt; line-height: 1.2 }
   P::first-letter { font-size: 200%; font-weight: bold; float: left }
   SPAN            { text-transform: uppercase }
  </STYLE>
 </HEAD>
 <BODY>
  <P><SPAN>The first</SPAN> few words of an article
    in The Economist.</P>
 </BODY>
</HTML>

This example might be formatted as follows:

The fictional tag sequence is:

<P>
  <SPAN>
    <P::first-letter>
      T
    </P::first-letter>he first
  </SPAN>
  few words of an article in the Economist.
</P>

Note that the ::first-letter pseudo-element tags abut the content (i.e., the initial character), while the ::first-line pseudo-element start tag is inserted right after the start tag of the block element.

3. Highlight Pseudo-elements

3.1. Selecting Highlighted Content: the ::selection, ::inactive-selection, ::spelling-error, and ::grammar-error pseudo-elements

The highlight pseudo-elements represent portions of a document that have been given a particular status and are typically styled differently to indicate that status to the user. For example, selected portions of the document are typically highlighted (given alternate background and foreground colors, or a color wash) to indicate their selected status. The following highlight pseudo-elements are defined:

::selection

::inactive-selection

The ::selection and ::inactive-selection pseudo-elements represent the portion of a document that has been selected as the target or object of some possible future user-agent operation(s). They apply, for example, to selected text within an editable text field, which would be copied by a copy operation or replaced by a paste operation.

::selection applies to active selections, whereas ::inactive-selection applies to inactive selections (e.g. when the document window is inactive and therefore not receiving events).

::spelling-error

The ::spelling-error pseudo-element represents a portion of text that has been flagged by the user agent as misspelled.

::grammar-error

The ::grammar-error pseudo-element represents a portion of text that has been flagged by the user agent as grammatically incorrect.

The highlight pseudo-elements do not necessarily fit into the element tree, and can arbitrarily cross element boundaries without honoring its nesting structure.

The following addition is made to the default UA stylesheet:

::spelling-error { text-decoration-line: spelling-error; }
::grammar-error { text-decoration-line: grammar-error; }

Note: A future level of CSS may introduce ways to create custom highlight pseudo-elements.

3.2. Styling Highlights

The highlight pseudo-elements can only be styled by a limited set of properties that do not affect layout. The following properties apply to the highlight pseudo-elements:

• color
• background-color
• cursor
• caret-color
• text-decoration and its associated properties
• text-shadow
• stroke-color, fill-color, and stroke-width

Are there any other properties that should be included here?

The color property specifies the color of both the text and all line decorations (underline, overline, line-through) and emphasis marks (text-emphasis) applied to the text by the originating element and its ancestors and descendants.

Note: Historically (and at the time of writing) only color and background-color have been interoperably supported.

3.3. Area of a Highlight

For each type of highlighting (see §3.1 Selecting Highlighted Content: the ::selection, ::inactive-selection, ::spelling-error, and ::grammar-error pseudo-elements) there exists a single highlight overlay for the entire document, the active portions of which are represented by the corresponding highlight pseudo-element. Each box owns the piece of the overlay corresponding to any text or replaced content directly contained by the box.

• For text, the corresponding overlay must cover at least the entire em box and may extend further above/below the em box to the line box edges. Spacing between two characters may also be part of the overlay area, in which case it belongs to the innermost element that contains both characters and is selected when both characters are selected.
• For replaced content, the associated overlay must cover at least the entire replaced object, and may extend outward to include the element’s entire content box.
• The overlay may also include other other areas within the border-box of an element; in this case, those areas belong to the innermost such element that contains the area.

See F2F minutes, dbaron’s message, Daniel’s thread, Gecko notes, Opera notes, Webkit notes

Not sure if this is the correct way of describing the way things work.

3.4. Cascading and Per-Element Highlight Styles

Each element draws its own active portions of the highlight overlays, which receives the styles specified by the corresponding highlight pseudo-element styles for which that element is the originating element. When multiple styles conflict, the winning style is determined by the cascade. When any supported property is not given a value by the cascade, it’s value is determined by inheritance from the corresponding highlight pseudo-element of its originating element’s parent element (regardless of whether that property is an inherited property).

p::selection      { color: yellow; background: green; }
p > em::selection { color: orange; }
em::selection     { color:    red; }

<p>Highlight this <em>and this</em>.</p>

Authors wanting multiple selections styles should use :root::selection for their document-wide selection style, since this will allow clean overriding in descendants. ::selection alone applies to every element in the tree, overriding the more specific styles of any ancestors.

::selection          { background: blue; }
p.warning::selection { background:  red; }

<p class="warning">Some <strong>very important information</strong></p>

:root::selection     { background: blue; }
p.warning::selection { background:  red; }

The UA should use the OS-default highlight colors for ::selection when neither color nor background-color has been specified by the author.

Note: This paired-cascading behavior does not allow using the normal cascade (i.g. :root::selection rules in the UA style sheet) to represent the OS default selection colors. However it has been interoperably implemented in browsers and is thus probably a Web-compatibility requirement.

3.5. Painting the Highlight

Each highlight pseudo-element draws its background over the corresponding portion of the highlight overlay, painting it immediately below any positioned descendants (i.e. just before step 8 in CSS2.1§E.2). The ::selection overlay is drawn over the ::spelling-error overlay which is drawn over the ::grammar-error overlay.

A highlight pseudo-element also suppresses the drawing of any selected text (and any text decorations applied to that text). Instead the topmost active highlight overlay redraws that text (and its decorations) over the highlight overlay backgrounds using its own color, with currentColor on its color property representing the color of the next highlight pseudo-element layer below, falling back finally to that of the originating element (the colors that would otherwise be used). Any text decorations introduced by each highlight pseudo-element are stacked in the same order as their backgrounds over the text’s original decorations and are all drawn, in their own colors.

Note: The element’s own text decorations (both line decorations and emphasis marks) are thus drawn in the pseudo-element’s own color when that is not currentColor, regardless of their original color or fill specifications.

What should happen with text shadows? Drawing them in their original color is disconcerting if that color is not a shade of gray. Maybe if the overlay has a background, suppress any text shadows below it?

For non-replaced content, the UA must honor the color and background-color (including their alpha channels) as specified. However, for replaced content, the UA should create a semi-transparent wash to coat the content so that it can show through the selection. This wash should be of the specified background-color if that is not transparent, else of the specified color; however the UA may adjust the alpha channel.

3.6. Security and Privacy Considerations

Because the styling of spelling and grammar errors can leak information about the contents of a user’s dictionary (which can include the user’s name and even includes the contents of his/her address book!) UAs that implement ::spelling-error and ::grammar-error must prevent pages from being able to read the styling of such highlighted segments.

4. Tree-Abiding Pseudo-elements

Tree-abiding pseudo-elements always fit within the box tree. They inherit any inheritable properties from their originating element; non-inheritable properties take their initial values as usual. [CSS3CASCADE]

4.1. Generated Content Pseudo-elements: ::before and ::after

When their computed content value is not none, these pseudo-elements generate boxes as if they were immediate children of their originating element, and can be styled exactly like any normal document-sourced element in the document tree.

::before

Represents a styleable child pseudo-element immediately before the originating element’s actual content.

::after

Represents a styleable child pseudo-element immediately after the originating element’s actual content.

p.note::before { content: "Note: " }

As with the content of regular elements, the generated content of ::before and :after pseudo-elements may be included in any ::first-line and ::first-letter pseudo-elements applied to its originating element.

4.2. List Markers: the ::marker pseudo-element

The ::marker pseudo-element represents the automatically generated marker box of a list item. (See display: list-item.)

The following CSS properties apply to a ::marker pseudo-element:

• all font properties (see [CSS3-FONTS])
• the color property (see [CSS3COLOR])
• the text-combine-upright property (see [CSS3-WRITING-MODES])

It is expected that future specifications will extend this list of properties; however at the moment marker box layout is not fully defined, so only these properties are allowed.

UAs must add the following rule to their default style sheet:

::marker { font-variant-numeric: tabular-nums; }

4.3. Placeholder Input: the ::placeholder pseudo-element

The ::placeholder pseudo-element represents placeholder text in an input field: text that represents the input and provides a hint to the user on how to fill out the form. For example, a date-input field might have the placeholder text “YYYY/MM/DD” to clarify that numeric dates are to be entered in year-month-day order.

Note: There also exists a :placeholder-shown pseudo-class, which applies to (real) elements while they are showing placeholder text, and can be used to style such elements specially. ::placeholder specifically selects a pseudo-element representing the placeholder text, and is thus relatively limited in its abilities.

All properties that apply to the ::first-line pseudo-element also apply to the ::placeholder pseudo-element.

In interactive media, placeholder text is often hidden once the user has entered input; however this is not a requirement, and both the input value and the placeholder text may be visible simultaneously. The exact behavior is UA-defined. Note that in static media (such as print) placeholder text will be present even after the user has entered input.

Authors seem to want text-align on the list of supported properties. See e.g. comments here.

Note: It’s been requested that ::placeholder also refer to a placeholder which has a corresponding element in the element tree. It’s not clear how this should work, but it may be worth doing. See Issue 2417.

5. Overlapping Pseudo-element Interactions

<style>
p { color: red; font-size: 12pt }
p::first-letter { color: green; font-size: 200% }
p::first-line { color: blue }
</style>

<p>Some text that ends up on two lines</p>

<p>
  <p::first-line>
    <p::first-letter>
      S
    </p::first-letter>
    ome text that
  </p::first-line>
  ends up on two lines
</p>

6. Additions to the CSS Object Model

6.1. Interface CSSPseudoElement

The CSSPseudoElement interface allows pseudo-elements to be event targets.

[Exposed=Window]
interface CSSPseudoElement : EventTarget {
    readonly attribute CSSOMString type;
    readonly attribute Element element;
};

The type attribute is a string representing the type of the pseudo-element. This can be one of the following values:

‘::before’

The pseudo-element was created before the element’s contents.

‘::after’

The pseudo-element was created after the element’s contents.

The element attribute is the originating element of the pseudo-element.

Note: This interface may be extended in the future to other pseudo-element types and/or to allow setting style information through a CSSStyleDeclaration style attribute. The current functionality is limited to that which is needed to support [web-animations-1].

6.2. Addition to the Element interface

A new method is added to the Element interface to retrieve pseudo-elements created by a given element for a given type:

partial interface Element {
  CSSPseudoElement? pseudo(CSSOMString type);
};

The pseudo(CSSOMString type) method is used to retrieve the CSSPseudoElement instance created by the element of the type matching type. Its return value is a CSSPseudoElement, potentially null if no such pseudo-element exists.

Pseudo-elements of unsupported types are considered to simply not exist; unrecognized type values are not an error.

The identity, lifetime, and nullness of the return value (and potential error cases) of the pseudo() method is still under discussion. See Issue 3607 and Issue 3603.

7. Compatibilitiy Syntax

For compatibility with existing style sheets written against CSS Level 2 [CSS21], user agents must also accept the previous one-colon notation (:before, :after, :first-letter, :first-line) for the ::before, ::after, ::first-letter, and ::first-line pseudo-elements.

Changes

Changes since the 7 June 2016 Working Draft include:

• Specified spelling-error and grammar-error with ::spelling-error and ::grammar-error in the UA stylesheet.
• Redefined value propagation between parent/child highlight pseudo-elements to use inheritance rather than cascading. See discussion in and linked from Issue 2474.
• Refined list of supported properties for highlight pseudo-elements, e.g. adding stroke-color/fill-color, removing outline, etc.
• Clarified how text and text decoration colors are painted for highlight pseudo-elements.
• Added the element attribute to the CSSPseudoElement interface.
• Changed the values of the type attribute on the CSSPseudoElement interface to match the corresponding pseudo-elements.
• Reduced supported types of CSSPseudoElement and removed unimplemented style attribute.
• Changed window.getPseudoElements(elem,type) to Element.pseudo(type).
• Miscellaneous minor clarifications and fixes.

Acknowledgements

The editors would like to thank the following individuals for their contributions, either during the conception of the specification or during its development and specification review process: Tab Atkins, David Baron, Razvan Caliman, Chris Coyier, Anders Grimsrud, Vincent Hardy.