dmitryf/elements — pull request #37 by relu (2012-12-03 22:34:22+00:00)
Title: Added background-clip as a mixin

I think it is safe enough to use the standard syntax for
-moz-background-clip, since the old one was supported only until Firefox
3.6 (Gecko 1.9.2).

https://developer.mozilla.org/en-US/docs/CSS/background-clip#Browser_compatibility