The CSSWG just discussed contain:style and resolved on the following:
* RESOLVED: remove style containment from keywords that aren't 'style'
* RESOLVED: mark 'style' at risk
* RESOLVED: Republish CR of css-contain once edits are in

https://github.com/w3c/csswg-drafts/issues/3280#issuecomment-467588765

For our initially shipping version of css containment, we're going to drop support for style from our CSS parser right now (and remove it from the content and strict keywords, per the first CSSWG resolution above).

Filing this bug on doing that.

We're not sure we're going to ship support for this value, and it's been marked
as 'at-risk' in the spec, so let's stop pretending we support it in the parser.

Spec issue:
https://github.com/w3c/csswg-drafts/issues/3280

Try run, just to be sure I didn't miss something:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=8c4016c6e0f88d93b883157563191d54e720acb5

Looks like we need some annotations and/or adjustments for the css/css-contain/parsing/contain-computed.html web platform test.

Try run with the wpt tests (for parsing contain:style [xyz]) annotated as failing:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=1ebe798ec1b4d219f34898cf7c41307a0c82eeb8

Once more with annotations for another test that I missed until now (contain-valid.html):
https://treeherder.mozilla.org/#/jobs?repo=try&revision=c08b60ce1749c65714a52299253bf3d25c169cae

https://hg.mozilla.org/mozilla-central/rev/9f04204160ff