There’s a broader point here about declarative design:
Setting very specific values may feel like you’re in more control, but you’re actually rescinding control by introducing fragility in the form of overly-specific CSS.
An excellent explanation of the new leading-trim and text-edge properties in CSS, complete with an in-depth history of leading in typography.
(I’m very happy to finally have a permanent link to point to about this, rather than a post on Ev’s blog.)
This abuse of the !important declaration in Firefox’s user-agent stylesheet was driving me crazy recently. Roger proposes a CSS patch, but this is really something that needs to be fixed in the browser.