The XLink spec clearly states that no href is required to have a valid XLink. The check for an href attribute in the IsSimpleXLink method in nsCSSStyleSheet is incorrect and should be removed - the type is enough.
Extremely simple to fix: remove two lines of code to not check for href.
Fix slipped in with bug 38026... less risk to leave it than to back it out.
Using the testcase from 12766 with modification, verified bug fixed.