[masking-1] describe the prefixed -webkit-mask-* properties

[dbaron]

The masking spec doesn't currently describe the -webkit-mask-* properties. However, these prefixed properties are shipping across multiple engines (I believe all of Chromium, Gecko, and WebKit) and may not be in a position that they can be removed from the platform. (That said, things could shift once Chromium ships the unprefixed properties, which should happen very soon.)

@progers wrote a document describing the syntax differences between the prefixed and unprefixed properties, although I don't know if many or all of these differences are implemented in Gecko (cc @emilio).

It seems like if these properties are going to be a permanent part of the platform we should probably specify them.

[emilio]

All the webkit-mask properties Gecko supports are just aliased to the standard properties (so we don't support the syntax differences mentioned in the doc). It'd be nice for Chrome / WebKit to try to get away with that if they can. I have never seen a compat issue due to us not supporting the non-standard values.