Sync web-platform-tests PR 15871 into mozilla-central (this bug is closed when the sync is complete).

PR: https://github.com/web-platform-tests/wpt/pull/15871
Details from upstream follow.

Anne van Kesteren <annevk@annevk.nl> wrote:

HTML: mutating the style element

I created this for https://github.com/whatwg/dom/pull/732#discussion_r266083085 since I didn't really trust the current abstractions we have in place. And we also haven't tested them as I traced in https://freenode.logbot.info/whatwg/20190315#c2058477. So this is an initial test before I call it a week to show there's a problem with the standard (all browsers fail this test and change the style sheet upon insertion of a Comment node).

cc @domenic @nox

CI Results

Ran 13 Firefox configurations based on mozilla-central, and Firefox, Chrome, and Safari on GitHub CI

Total 1 tests and 4 subtests

Status Summary

Firefox

OK : 1
PASS: 4

Chrome

OK : 1
PASS: 4

Safari

OK : 1
PASS: 4

Links

Gecko CI (Treeherder)
GitHub PR Head
GitHub PR Base

Landed: https://hg.mozilla.org/integration/autoland/rev/e5bae276c12dcdd6e54b0fed97f165f83a274308

https://hg.mozilla.org/mozilla-central/rev/e5bae276c12d