[whatwg] Support for <link rel="stylesheet" crossorigin>

[Boris Zbarsky]

I just added support for <link rel="stylesheet" crossorigin> in 
Gecko.[1]  Such links are subject to CORS checks if the load is 
cross-site, and the sheet load will fail if the CORS check fails.  If 
the CORS check succeeds, script in the page will be able to script the 
cross-site stylesheet.

This allows stylesheets served from a CDN to be scripted by the page if 
desired, so I would like to propose that we add this feature to the spec.

An open issue: what to do about @import?  I haven't done anything magic 
here yet.  Inheriting the CORS mode from the importing sheet is a bit 
weird, and I wasn't quite sure I wanted to make CSS syntax changes at 
this time.

-Boris

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=732209