Intent to ship: font-size: xxx-large

[Emilio Cobos Álvarez]

Summary:

Expose the xxx-large value for the font-size property, as resolved in
https://github.com/w3c/csswg-drafts/issues/3907.

This value was previously accessible only via `<font size="7"></font>`
in HTML.

Sending an intent on behalf of the patch author.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1553545

Standard:
https://drafts.csswg.org/css-fonts-4/#valdef-font-size-absolute-size

Platform coverage: All

Preference: None

DevTools bug: N/A

Other browsers:

Blink: Fixed on trunk
(https://bugs.chromium.org/p/chromium/issues/detail?id=966249)
WebKit: Proposed the feature, but hasn't implemented yet AFAIK.

web-platform-tests: They exist, and a new reftest is added. See the patch.

Secure contexts: Not restricted, as all the other CSS font-size keywords.

Is this feature enabled by default in sandboxed iframes? Yes

-- Emilio