Intent to ship: CSS overflow:clip

[Mats Palmgren]

Summary:
We intend to ship CSS overflow:clip in v81, which is the standardized
version of a prefixed value (-moz-hidden-unscrollable) that we already
ship. The prefixed value becomes an alias for the new value.
The standardized value is mostly compatible, but there are some
differences. Most notably, we now support clipping in one axis
and overflowing in the other (i.e. 'clip' can be combined with
'visible' in the overflow-x/y properties).

Bugs:
https://bugzilla.mozilla.org/show_bug.cgi?id=1531609
https://bugzilla.mozilla.org/show_bug.cgi?id=1635473

Standard:
https://drafts.csswg.org/css-overflow-3/#valdef-overflow-clip
https://github.com/w3c/csswg-drafts/issues/1971

Platform coverage:
All

Preference:
None. We've shipped the prefixed version of this value since forever
and any web-compat issues are unlikely.

DevTools bug:
N/A

Other browsers:
It's not implemented in Blink/WebKit yet:
https://bugs.chromium.org/p/chromium/issues/detail?id=966283
https://bugs.webkit.org/show_bug.cgi?id=198230
Blink appears to show positive signals in:
https://github.com/w3c/csswg-drafts/issues/1971

Web-platform-tests:
Yes, some already exist and I've added more.

Secure contexts:
This will not be restricted to secure contexts, like many other CSS features.

Is this feature enabled by default in sandboxed iframes?
Yes, like many other CSS features.


Credit also goes to Jeremy Ir who implemented the initial version of
the style system changes for this feature.


/Mats

[Mats Palmgren]