[css-backgrounds] Standardize "background-clip: text"

Hi,

We are currently implementing "-webkit-background-clip: text" [1] for web
compatibility. This property (with the "text" value) is specced in the
Compatibility standard [2] at this moment. We found that, making
"-webkit-background-clip" a separate property instead of an alias of the
unprefixed "background-clip" adds complexity to implementation (parsing and
serialization).

Edge has supported "background-clip: text", and I suppose WebKit does, too.
Blink does not accept "text" for "background-clip", however, if you set
"-webkit-background-clip: text", the computed value of "background-clip"
would be "text" as well (which seems to be rather broken somehow).

Given these, it seems we should probably reconsider the decision about this
value on background-clip [3]. I agree that the design of the value is
terrible, but as it has been used widely, and all browser vendors have
implemented it, I think it makes sense to put it into css-backgrounds spec
as part of background-clip property.


[1] https://bugzilla.mozilla.org/show_bug.cgi?id=759568
[2] https://compat.spec.whatwg.org/#the-webkit-background-clip-property
[3] https://lists.w3.org/Archives/Public/www-style/2012May/1228.html

- Xidorn