1254424 - Animations using em-based units should respond to changes in the font-size on the parent

Status: RESOLVED WONTFIX

(Core :: CSS Parsing and Computation, defect)

Description

[Brian Birtles (:birtles)]

Attachment: Test case

In the attached test case, two elements have a CSS animation that animates the margin-left from '0em' to '10em'. After the first iteration, the font-size is updated.

For the green box, the font-size on the element itself is updated.
For the red box, the font-size on the parent element is updated.

Expected results:
After the first iteration both animations start animating "further" to the right, due to the increased font-size.

Actual results:
Only the green box (the element whose specified style was updated, as opposed to its parent's) updates its animation. However, some actions such as opening DevTools inspector can cause the red box to suddenly update.


This is a subset of the work needed to bring our CSS animations implementation into line with the behavior resolved by the CSS WG regarding liveness[1] but which has yet to be edited into the spec[2].

Bug 1245748 should make us start storing the specified values so that we can re-resolve them into (computed) StyleAnimationValues when we detect a situation where the meaning of context-sensitive values might have changed.

[1] https://lists.w3.org/Archives/Public/www-style/2015Oct/0226.html item 1a
[2] https://github.com/w3c/csswg-drafts/issues/66

Comment 1

[Hiroyuki Ikezoe (:hiro)]

Note that in stylo this bug will be fixed by bug 1350754.

Comment 2

[Chris Mills [:cmills]]

Marking as part of documentation plan for intentional stylo differences

Comment 3

[Chris Mills [:cmills]]

Documented:

https://developer.mozilla.org/en-US/docs/Web/CSS/animation#Quantum_CSS_notes
https://developer.mozilla.org/en-US/docs/Web/CSS/font-size#Quantum_CSS_notes
https://developer.mozilla.org/en-US/Firefox/Releases/57#Quantum_CSS_notes

Comment 4

[Hiroyuki Ikezoe (:hiro)]

Closing this works as expected on our new style system (i.e. stylo), and we already dropped the old style system.