Intent to ship: aspect-ratio property

[Boris Chiou]

I intend to turn the preferred aspect ratio, `aspect-ratio`
<https://drafts.csswg.org/css-sizing-4/#aspect-ratio> property, on by
default on all platforms after the last bug got landed (i.e. bug 1646098
<https://bugzilla.mozilla.org/show_bug.cgi?id=1646098>), so the most
possible version should be Firefox 88. It has been developed behind the
layout.css.aspect-ratio.enabled preference. The spec should be stable
enough and CSSWG agrees to ship this feature (csswg-drafts#5598
<https://github.com/w3c/csswg-drafts/issues/5598>).

Status in other browsers:
Google Chrome: Ship already last Oct. (Chromium intent to ship mail
<https://groups.google.com/a/chromium.org/g/blink-dev/c/TF41VMfLhMI>)
Safari: Implementing this feature, and has supported this in Safari
Technology Preview.

*Bug to turn on by default*:
https://bugzilla.mozilla.org/show_bug.cgi?id=1672073

There are bunch of wpts already in
https://github.com/web-platform-tests/wpt/tree/master/css/css-sizing/aspect-ratio

This feature was previously discussed in this "Intent to prototype"
thread:
https://groups.google.com/g/mozilla.dev.platform/c/mWViuwFw8b8/m/HVHJ8yAnAAAJ

Please let me know if you have any concerns.

Thank you.