*Summary: *Per the result of this spec issue, csswg#4441
<https://github.com/w3c/csswg-drafts/issues/4441>, we would like to start
the CSS transitions on CSS properties with discrete animation types. The
new definition of transitionable is:
     property values are transitionable *unless* they have an animation
type that is not animatable. Values with a discrete animation type *are*
transitionable, and flip at 50% progress (p = 0.5).

However, the "all" keyword continues to expand only to all transitionable
properties whose animation type is not discrete.

Note that, even if the animation type is by computed value, but its
interpolation falls back to discrete (e.g. from "10px" to "auto"), we also
treat it as a discrete animation case, and so we still start a transition
for it, if the user specifies this CSS property in "transition-property".

*Bug:* https://bugzilla.mozilla.org/show_bug.cgi?id=1805727
*Specification*:
https://drafts.csswg.org/css-transitions-2/#transition-property-property
*Standards Body:* CSSWG
*Platform coverage:* all

*Preference*: "*layout.css.transition-discrete.enabled*". Will enable it on
Nightly for now, so wpt.fyi can show its result for interop-2023, and it
makes us easier to catch issues on try server.
*DevTools bug*: No needed
*Link to standards-positions discussion*:
https://github.com/mozilla/standards-positions/issues/763

*Other browsers*:

   - Blink: Implemented this already. Not shipped.
   https://bugs.chromium.org/p/chromium/issues/detail?id=1399631
   - WebKit: Not shipped or implemented. I didn't see any related WebKit
   bug for now. Only standards-positions:
   https://github.com/WebKit/standards-positions/issues/148

*web-platform-tests*: Basically, every spec with animation tests should
includes CSS transitions and these tests should test this feature already
(e.g. the flex-box test
<https://searchfox.org/mozilla-central/rev/31f5847a4494b3646edabbdd7ea39cb88509afe2/testing/web-platform/tests/css/css-flexbox/animation/discrete-no-interpolation.html#10-14>
uses "test_no_interpolation()" to test if two values are not interpolated
and so the property value flips at 50%, for both transitions and
animations). And perhaps this one:
css/css-transitions/all-with-discrete.tentative.html
<https://searchfox.org/mozilla-central/rev/31f5847a4494b3646edabbdd7ea39cb88509afe2/testing/web-platform/tests/css/css-transitions/all-with-discrete.tentative.html>
is the most relevant one.


Thanks.

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