Correct misinformation regarding quotes in CSS attribute selectors

The CSS 2.1 spec doesn’t show quotes “as optional” at all. Whether they can or cannot be removed depends entirely on the attribute value. See <http://mathiasbynens.be/notes/unquoted-attribute-values#css> for details, or <http://mothereff.in/unquoted-attributes> for a tool that probably visualizes this more clearly.
Merge pull request #37 from mathiasbynens/gh-pages

Correct misinformation regarding quotes in CSS attribute selectors