Intent to ship: CSS properties text-decoration-skip-ink, text-decoration-thickness, and text-underline-offset

[Daniel Holbert]

As of today (Sept 12 2019), I've turned on support for the CSS properties
text-decoration-skip-ink, text-decoration-thickness, and
text-underline-offset, on all platforms.

These features have been developed behind the preferences
"layout.css.text-underline-offset.enabled",
"layout.css.text-decoration-thickness.enabled", and
"layout.css.text-decoration-skip-ink.enabled".

Other UAs shipping these features:
* Safari 12.1 (and newer) supports all these features
* Chrome/Blink supports text-decoration-skip-ink, and they have
https://bugs.chromium.org/p/chromium/issues/detail?id=785230 open on the
other properties.

Bug to enable by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1573631

Note that these properties were already enabled by default in Nightly and
early-beta, as of Firefox 70. I intend to uplift bug 1573631's patch to
beta, to remove that restriction, as part of the Firefox 70 beta cycle, so
that this will ship in Firefox 70.

These features were previously discussed in these intent-to-prototype
threads:
https://groups.google.com/d/msg/mozilla.dev.platform/Xsts-2ORpRY/j96vHsIRAAAJ
https://groups.google.com/d/msg/mozilla.dev.platform/VWcV5QUzJF0/O9hET80TAgAJ
https://groups.google.com/d/msg/mozilla.dev.platform/iwkhLi-2mxw/2aJPi80TAgAJ