As mentioned in bug 1341077 and bug 1341078: We currently have tests in layout/reftests/w3c-css/ which also appear in testing/web-platform/tests/. It would be good to get rid of the duplication, given it frequently results in people not noticing tests that already exist, and get rid of the duplicate tooling for syncing (I imagine layout/reftests/w3c-css/imported/ may well be out-of-date with upstream changes, too). This would allow us to resolve https://github.com/w3c/web-platform-tests/issues/8615.

David: are the dependencies of this still an accurate listing of things you think block this?

Sort of, I guess. I'm not sure that bug 1263568 was really exhaustive, since I was to some degree just filing issues in support of bug 1263568 comment 4. But fixing those is likely to be close enough that I'd be willing to convert tests given a commitment to work on further regressions that turn up.

We also don't run the web-platform-test reftests in the css/ subdirectory on any platform other than Linux: https://searchfox.org/mozilla-central/rev/924e3d96d81a40d2f0eec1db5f74fc6594337128/testing/mozharness/scripts/web_platform_tests.py#217-218 We need to run them on Mac, Windows, and Android. Bug 1481723 covers Mac and Windows.

A bunch of the work for this appears to be happening in bug 1490969.