title: CodeMirror Dynamic Syntax Validation
body: <p>Been trying to decide between using CodeMirror or Ace editor. I've been leaning towards CodeMirror, however there's one feature of Ace that I really like and that is how it does syntax validation. So as I'm typing there can appear a warning or error icon in the left gutter area beside the line number, and when I hover over it it gives me a little description.</p>
  
  <p>Is there any way to get this functionality in CodeMirror? Specifically, I'm using the css mode for CodeMirror.</p>
  
  <p>It'd also be nice to be able to add in my own custom validation.</p>
  
  <p>Thanks.</p>