title: CSS attribute selectors: The rules on quotes (", ' or none?)
body: <p>This question has been bugging me for a while now. When writing a CSS selector that compares against an element's attribute like so.</p>
  
  <pre><code>a[rel="nofollow"]
  </code></pre>
  
  <p>I never know what I should be doing with the quotation marks. Are they really necessary?</p>
  
  <p>Basically, what is the specification for this because I can't find it on the web site.</p>
  
  <p>Are all of these considered valid?</p>
  
  <pre><code>a[rel="nofollow"]
  a[rel='nofollow']
  a[rel=nofollow]
  </code></pre>