title: Allowed characters in CSS 'content' property?
body: <p><a href="http://css-tricks.com/css-content/" rel="nofollow noreferrer">I've read that</a> we must use <a href="https://stackoverflow.com/a/4476362/1071413">Unicode values inside the <code>content</code> CSS property</a> i.e. <code>\</code> followed by the special character's hexadecimal number.</p>
  
  <p>But what characters, other than alphanumerics, are actually allowed to be placed as is in the value of <code>content</code> property? (Google has no clue, hence the question.)</p>