mdo/code-guide — pull request #37 by mathiasbynens (2014-03-03 10:15:14+00:00)
Title: 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 for details, or for a tool that probably visualizes this more clearly.