[Tracking Requested - why for this release]:

Central as beta:
https://treeherder.mozilla.org/#/jobs?repo=try&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel%2Crunnable&revision=c3e783a4e7b856e063c52412ff3e25b2f175f8c5&selectedJob=265124259

Failure logs:
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265124259&repo=try&lineNumber=1977
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265132081&repo=try&lineNumber=2576
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265132079&repo=try&lineNumber=2560
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265121463&repo=try&lineNumber=2417
https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=265121447&repo=try&lineNumber=3336

Failures:
TEST-UNEXPECTED-FAIL | /css/css-box/animation/padding-interpolation.html | padding interpolation - NotSupportedError: Animation to or from an underlying value is not yet supported.
TEST-UNEXPECTED-ERROR | /css/css-backgrounds/animations/border-color-interpolation.html | NotSupportedError: Animation to or from an underlying value is not yet supported.
TEST-UNEXPECTED-FAIL | /css/css-transforms/animation/transform-interpolation-006.html | transform interpolation - NotSupportedError: Animation to or from an underlying value is not yet supported.
TEST-UNEXPECTED-FAIL | /css/css-multicol/animation/column-count-interpolation.html | column-count interpolation - NotSupportedError: Animation to or from an underlying value is not yet supported.
TEST-UNEXPECTED-FAIL | /css/css-transforms/animation/transform-interpolation-006.html | transform interpolation - NotSupportedError: Animation to or from an underlying value is not yet supported.

Depends on D44938

Depends on D44939

Depends on D44940

https://hg.mozilla.org/mozilla-central/rev/cc83d7fe35d8
https://hg.mozilla.org/mozilla-central/rev/58011a2d3dbc
https://hg.mozilla.org/mozilla-central/rev/2d92a2fa764a
https://hg.mozilla.org/mozilla-central/rev/62f5b57b9a03

Today's beta-sim: https://treeherder.mozilla.org/#/jobs?repo=try&resultStatus=success%2Ctestfailed%2Cbusted%2Cexception&revision=cb853226098e88a353228488821cf30d3c106ad0&searchStr=android%2C7.0%2Cx86-64%2Copt%2Cweb%2Cplatform%2Ctests%2Ctest-android-em-7.0-x86_64%2Fopt-geckoview-web-platform-tests-e10s-6%2Cw%28wpt6%29&selectedJob=265317460

This is reappearing and I believe it's being caused by the changes in https://bugzilla.mozilla.org/show_bug.cgi?id=1578125#c30, more exactly: https://hg.mozilla.org/mozilla-central/rev/73d9659e7715
Brian could you please take a look at it?

Failures: https://treeherder.mozilla.org/#/jobs?repo=try&resultStatus=testfailed%2Cbusted%2Cexception&revision=3e07436268e322892f960f046e9dd1f0c555670f&searchStr=%28wpt&selectedJob=267871715

James, can we set a pref to be true for all WPT runs?

The issue is Blink checked in a common interpolation test function that uses a feature we have prefed-off on beta/release. That test function gets used by dozens of specs and every time someone uses it in a new spec we need to add another __dir__.ini to un-break beta.

I notice there is a root __dir__.ini but I don't see any prefs set in it, so I guess it's not the done thing?

Sure, put it in https://searchfox.org/mozilla-central/source/testing/profiles/web-platform/user.js

This pref is required by the common animation interpolation test function used
in many of the CSS test suites but currently we only enable it on Nightly. As a
result, each time a new spec adds animation tests, we get failures on our beta
simulations. Rather than turn this on for each directory it is used in each
time a new failure occurs, this patch makes us set it globally.

This patch also removes a number of instances where we set this pref locally.
It only removes the instances that were specifically added due to the
introduction of the common CSS interpolation test function. The existing
instances remain untouched since they serve as documentation of other places
where we are specifically testing that feature.

https://hg.mozilla.org/mozilla-central/rev/288f3414b425

Verified as fixed with this beta sim:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=5c682c9f151590c24a9f0226ae16a3abc4708d86&searchStr=wpt

Bugbug thinks this bug is a regression, but please revert this change in case of error.