WPT import https://crrev.com/c/1184961 introduced new failures in external/wpt/css:

List of new failures:
external/wpt/css/css-transitions/CSSTransition-startTime.tentative.html new failing tests: https://crrev.com/c/1184961/4/third_party/WebKit/LayoutTests/external/wpt/css/css-transitions/CSSTransition-startTime.tentative-expected.txt
[ Mac-mac10.10 ] external/wpt/css/css-transitions/CSSTransition-startTime.tentative.html new failing tests: https://crrev.com/c/1184961/4/third_party/WebKit/LayoutTests/platform/mac-mac10.10/external/wpt/css/css-transitions/CSSTransition-startTime.tentative-expected.txt
[ Mac-mac10.11 ] external/wpt/css/css-transitions/CSSTransition-startTime.tentative.html new failing tests: https://crrev.com/c/1184961/4/third_party/WebKit/LayoutTests/platform/mac-mac10.11/external/wpt/css/css-transitions/CSSTransition-startTime.tentative-expected.txt

This import contains upstream changes from d829f205b7ac265c78a45ba75863baa404d4ce72 to d2fa1bdc90e7a56f16f28d096de3a6c22f711033:
Shadow DOM: test more non-safelisted elements in attachShadow: https://github.com/web-platform-tests/wpt/commit/d2fa1bdc90e7a56f16f28d096de3a6c22f711033
Merge pull request #12613 from dhdavvie/dhdavvie/h2-server: https://github.com/web-platform-tests/wpt/commit/79f422aac425704b0276c22fa2bc072ab59494dc
Added some comments to better explain some APIs and functionality. Minor touchup with regard to how much data is read from the socket.: https://github.com/web-platform-tests/wpt/commit/ae6f0ea4b62a9d6bfe84f73b5fd7d02a5d7e9d23
Part 7: Tests.: https://github.com/web-platform-tests/wpt/commit/360acfbd672e986725c6f3aead60df8dfa93f36f [affecting this directory]
[wptrunner] Report unstable tests with exit code: https://github.com/web-platform-tests/wpt/commit/a59e0f3bbf4d218c688492d63a33a519e63192eb
Fetch: use more any.js functionality in CORP test: https://github.com/web-platform-tests/wpt/commit/adc30f943e5b46b3c49efbfe7d473a9599781006
Add tolerance for time comparison in playing-an-animation.html: https://github.com/web-platform-tests/wpt/commit/9e2420163b8c08b2b0f245d45e4e30e20c244c69
Use .any.js features for SAB nested workers test: https://github.com/web-platform-tests/wpt/commit/ba5c61c1d3424e107dfc6958c5628a122657ac7f
Typo: Ovveride → Override (#12604): https://github.com/web-platform-tests/wpt/commit/5fd720689e268ab946873e114e74f8f9af7d95e3
Don't do whitespace re-attachment for ::first-letter.: https://github.com/web-platform-tests/wpt/commit/05f0eb425c0283275fae1fc8f2af437ecf7bf49b [affecting this directory]
Merge pull request #12594 from gsnedders/fixup-7633: https://github.com/web-platform-tests/wpt/commit/a6c7a82230cf5c35acc384350bcd61af5ea638a5
Fieldset should support CSS Grid: https://github.com/web-platform-tests/wpt/commit/df7128b758445f8f67da701810da86c95f8520f9
Move the enabled state to the sheet instead of sharing it.: https://github.com/web-platform-tests/wpt/commit/41b280a0da8868386f6fdb0e16b8fbfb2cfee83b [affecting this directory]
[wdclient] Fix handling of "invalid session id" error.: https://github.com/web-platform-tests/wpt/commit/1b277ddedae1574fd37d76f91716780ce4e9dce3
Sync Mozilla tests as of https://hg.mozilla.org/mozilla-central/rev/a955df76e2b636889a1c37a7863739d6306520eb .: https://github.com/web-platform-tests/wpt/commit/f93b9e7f735ba39297971483c4ceccbbbf216249 [affecting this directory]
Move files out of (the versioned) css/css-values-3: https://github.com/web-platform-tests/wpt/commit/d2e1294c4bfee6f4dc901ba1b2065b5257e3ebe5 [affecting this directory]
Remove executable bit in tools/third_party/h2/ (#12591): https://github.com/web-platform-tests/wpt/commit/f5e86c83e53b55916056ba7270646a8757a193c7
HTML: test fieldset's legend and painting order: https://github.com/web-platform-tests/wpt/commit/fa88a22846b422d4128c9e6c049ad33fe675c2c9
tests for onpayerdetailchange (#11772): https://github.com/web-platform-tests/wpt/commit/906cac1dbf6e4fdcaa94bda8cf73c6344dce964d
Remove duplicated 'var's in urlsearchparams-constructor.any.js.: https://github.com/web-platform-tests/wpt/commit/b1db4409a550bb4a085ef9043f3e66cb44416f06
Add some missing 'var's to urlsearchparams-constructor.any.js.: https://github.com/web-platform-tests/wpt/commit/c6edf6dc6e80a48cf16014908fe0e30d6a821c34
HTML: test fieldset block formatting context: https://github.com/web-platform-tests/wpt/commit/1202ff97c023b490126b0c1283095c045800baee
Make CSSTransition startTime test timeout when waiting for transition events after a fixed number of frames: https://github.com/web-platform-tests/wpt/commit/0398567bac5d9bfde928cd200ac3f52c1acfcb7e [affecting this directory]
Add a test for resuming from paused: https://github.com/web-platform-tests/wpt/commit/517090a09ace429f257acd8bd6da9eec1faeabad
Prevent WPT from failing when all tests are skipped.: https://github.com/web-platform-tests/wpt/commit/a669cf82a8734022b11f4dee963c9b2f506dedd2