Intent to ship: CSS Mask Image

[Ku(顧思捷)CJ]

After bug 1251161 landed, we turned CSS mask image on by default on all
platforms. It has been developed behind the MOZ_ENABLE_MASK_AS_SHORTHAND
compile flag. Chrome is already shipping this in version 29.

(Original plan is to turn it on by FF 50, delay it to FF 53 is because of
some performance work, such as bug 1234485)

Related Bugs:
META: https://bugzilla.mozilla.org/show_bug.cgi?id=1224422
Blockers: https://bugzilla.mozilla.org/show_bug.cgi?id=1251161

Spec:
https://drafts.fxtf.org/css-masking-1/

Target release:
Firefox 53 (current Nightly).

[L. David Baron]

What is being proposed to ship is only part of the spec, section 7
(Positioned Masks):
https://drafts.fxtf.org/css-masking-1/#positioned-masks
These are the masking properties that are analogous to the
background image properties.

We have no work ongoing on section 8 of the spec, which covers the
border-image style CSS masks (mask-border-*). The priority of that
work isn't clear to me; I think it depends largely on how much
effort would be required given that we already have mask-image and
border-image. I think (based on caniuse, which isn't incredibly
clear) that WebKit and Chromium implement both parts, and Edge
doesn't implement either part.


For other parts of the spec (less related to "mask images"), see the
spec's metabug:
https://bugzilla.mozilla.org/showdependencytree.cgi?id=1312613&maxdepth=1&hide_resolved=0


I'm in support of shipping this work, assuming that the performance
concerns that delayed it have been addressed.

-David

--
𝄞 L. David Baron http://dbaron.org/ 𝄂
𝄢 Mozilla https://www.mozilla.org/ 𝄂
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offense.
- Robert Frost, Mending Wall (1914)