servo/servo — pull request #4084 by Ms2ger (2014-11-24 10:14:20+00:00)
Title: Panic if DOMTokenList#contains is called for an unparsed attribute.

Previously, if the attribute was not parsed into a token list, and the
tokens() method returned None, DOMTokenList#contains would silently return
false. This issue was encountered in
and took quite some time to
figure out.