title: What causes an xmllint warning?
body: <p>I'm currently writing a plugin for our integration server that uses libxml2's <code>xmllint</code> command line tool to validate XML files. According to the <a href="http://xmlsoft.org/xmllint.html" rel="nofollow">manual</a>, xmllint has a <code>--nowarning</code> option that suppresses warnings.</p>
  
  <p>Now, my question is rather simple, and I'm probably just missing something blatantly obvious, but what causes such a warning? It's kinda hard to parse the output if I don't know what exactly it looks like :-)</p>