Fix specificity issue for <embed hidden>

[zcorpan]

The embed[hidden] rule was intended to override 'display' but had lower specificity.

Fixes #9564.

• At least two implementers are interested (and none opposed):
  • See Fix specificity issue for <embed hidden> #9664 (comment)
• Tests are written and can be reviewed and commented upon at:
  • HTML: rendering of <embed hidden> and <embed hidden=until-found> web-platform-tests/wpt#41733
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=893946
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=238266
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• MDN issue is filed: N/A

(See WHATWG Working Mode: Changes for more details.)

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/rendering.html ( diff )

[domenic]

LGTM. Not quite sure what the multi-implementer interest status is for this but I'll trust you to figure that out before merging.

[zcorpan]

hidden=until-found is only implemented in Chromium, but it still passes the test, so it seems it somehow didn't have the bug that the spec had. WebKit and Gecko also pass the display part of the test, so I think we can treat this as a spec-only bug fix.

FYI @josepharhar