[Gecko Bug 1846635] [css-properties-values-api] Support <string> syntax

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D185203

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1846635 gecko-commit: 168ad53da9fdaa993c4f26b252848fb8e76169aa gecko-reviewers: emilio

[lilles]

There is no spec change landed yet afaik. Also, this changes an interop 2023 test which should formally be approved to be added through an interop github issue.

[zrhoffman]

Send a spec PR

PRed w3c/css-houdini-drafts#1104

and raise an interop issue?

Raised #web-platform-tests/interop#389

[jgraham]

@lilles Can you verify that this is OK to land now?