Make <attr-name> accept any CSS identifier

Make <attr-name> accept any CSS identifier rather than
rejecting those starting with minus.
With this change we do match attributes starting with minus
unless they are dynamically added.

Behavior matches Firefox.

Review-Url: https://codereview.chromium.org/1924313002
Cr-Commit-Position: refs/heads/master@{#391412}
5 files changed