Attributes need case-insensitive matching on getting

[annevk]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1165851#c23 for what is (likely, hopefully someone confirms my analysis) wrong with the current specification.

[annevk]

Note, the way this seems to be solved is simply lowercasing the argument to createAttribute(). That's certainly much more attractive.

[Ms2ger]

web-platform-tests/wpt#2198
web-platform-tests/wpt#2203