Accept unknown webkit pseudo-element.

Differential Revision: https://phabricator.services.mozilla.com/D4186

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1424106
gecko-commit: 988b2e838fd1c3c335169808b563ee6c07feeac9
gecko-integration-branch: autoland
gecko-reviewers: emilio

DOM: test :nth-child selector with whitespace in the argument

Merge pull request web-platform-tests#12681 from web-platform-tests/s…

…ync_91f298a913836e46d27482e189ce25475298196c

Merge pull request web-platform-tests#12681 from sync_91f298a913836e46d27482e189ce25475298196c

Revert "Merge pull request web-platform-tests#10344 from nox/var-gt-c…

…onst"

This reverts commit b80da5d, reversing
changes made to 603e277.

Fixup import of WebDriver helper functions

HTML: fixup for legend position: relative test

This test fails in Firefox because it doesn't leave a gap for the
legend, but this test isn't intended to test that.

HTML: Add margins to reference for fieldset translateZ test

HTML: fix a bug in img environment changes test

Fixes web-platform-tests#12656.

HTML: force layout in legend-dynamic-update.html

Stop any further transitions creation once after we got a transitione…

…nd for the width transition.

The transition property in this test is shorter than the period of chaging
parent viewport height so that it's possible that a new transition happens
after a transitionend event was dispatched.  The new transition will reduce
the element width (i.e. it's opposite direction of the first transition),
so the transitioned yellow box was smaller than the expected result.

Differential Revision: https://phabricator.services.mozilla.com/D4183

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1438814
gecko-commit: f433ed4ff1de863250d6ebf82b5bc5afcea9b6c7
gecko-integration-branch: mozilla-inbound
gecko-reviewers: birtles