Intent to ship: supports conditions for CSS imports

[CanadaHonk]

As of today, I intend to turn supports conditions for CSS imports (@import supports(...)) on by default on all platforms.

It has been developed behind the layout.css.import-supports.enabled preference, enabled in Nightly.

It is not supported in any other browsers at this time:
Chromium bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1380321

WebKit bug: Unfiled

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1830779

Standard: https://www.w3.org/TR/css-cascade-5/#conditional-import

This feature was previously discussed in this "Intent to prototype" thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/sWJkRYO58dc

[Sebastian Zartner]

For what it's worth, I've just filed a bug for WebKit at https://bugs.webkit.org/show_bug.cgi?id=256180.

Sebastian