title: Which Polymer lifecycle event should be used to modify contained elements before they render?
body: <p>I'm writing a <code>&lt;my-codeblock&gt;</code> element, in which I'd like to strip leading and trailing whitespace from the content. Of the <a href="http://www.polymer-project.org/docs/polymer/polymer.html#lifecyclemethods" rel="nofollow">polymer lifecycle events</a>, which is the best to use to traverse the contents of the custom element and modify them?</p>
  
  <p>I definitely want to get the modification done before the first paint, and it would be nice to help the polyfill/browser avoid extra work when distributing the nodes I'm going to modify into the shadow dom.</p>