In https://hg.mozilla.org/integration/autoland/rev/27209bcee14f#l3.1 , we removed some lines from the web-platform-test text-decoration-invalid.html, because the styles were now valid (due to the text-decoration shorthand suddenly accepting length-flavored syntax for its new text-decoration-thickness subproperty).

I didn't think of it during code review, but we really should add those lines back in the "valid" version of that testcase:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/css/css-text-decor/parsing/text-decoration-valid.html
(We should also add "from-font" to that "valid" test, since that is also now a valid value for text-decoration for the same reason.)

And we should adjust the .ini file for this test to enable the text-decoration-thickness pref for that testcase, so that it passes.

It looks like I actually shouldn't have removed any of this in the first place. It seems like there's some sort of bug where we don't properly serialize the default values for subproperties in text-decoration, so auto is intended to fail. I've added some extra cases and updated the ini file accordingly.

https://hg.mozilla.org/mozilla-central/rev/4678b2d85df4