Intent to unship -moz-border-*-colors from content pages.

[Emilio Cobos Álvarez]

Hi dev-platform@,

In bug 1417200, I intend to hide the -moz-border-top-colors,
-moz-border-bottom-colors, -moz-border-left-colors, and
-moz-border-right-colors from content pages.

The reasons for doing this are multiple:

* Non-standard properties.

* Kinda-weird, in the sense that the "border" shorthand resets it, but
the other border-<side> shorthands don't.

* Quoting jrmuizel:

> Supporting this is a pain for webrender. It would be nice not to
expose them to web content and then eventually remove from chrome.

* Typical use cases can be addressed with border-image in a
cross-browser way.

* It got a standard replacement in the latest draft of css-backgrounds
(making border-color take a list of colors instead of just one), so if
there's buy-in from other vendors we can expose that functionality
there, see [1] and [2].

I think it's nicer to remove these before other browsers implement that,
because otherwise we may be stuck with them forever, or make removing
these much more risky.
Thoughts?

-- Emilio

[1]: https://github.com/w3c/csswg-drafts/issues/1172#issuecomment-295565255
[2]: https://drafts.csswg.org/css-backgrounds-4/#propdef-border-color