Correct misinformation regarding quotes in CSS attribute selectors

The CSS 2.1 spec doesn’t show quotes around attribute values “as optional” at all. Whether they can or cannot be omitted 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.