CSS parser does not support -webkit-mask-size for -webkit-mask shorthand. It doesn't look correct. I would like to fix it, but will wait for comments if there is a reason for it. -webkit-mask-size itself is implemented and seems to work.
Created attachment 175813 [details] Patch
Comment on attachment 175813 [details] Patch r=me
Committed r136072: <http://trac.webkit.org/changeset/136072>