title: Chrome Extension - Content Script unable to find elements by class name
body: <p>I am attempting to access elements with a specific class name from a page using a content script of a Chrome extension.  So far the content script can successfully find an element with a specific id using document.getElementById(), but using document.getElementsByClassName() or jQuery's $(".className") yields no results.  For the sake of testing, I used 'header' as my class name and every website I ran the extension on resulted in an array length of 0.  Any ideas what I might be missing?  Here's what I have been testing with:</p>
  
  <pre><code>manifest.json
  =================
  {
      "name": "Sample Extension",
      "version": "0.0.1",
      "description": "Sample extension",
      "icons": {"128": "icon.png"},
      "permissions": [
          "tabs", "&lt;all_urls&gt;"
      ],
      "browser_action": {
          "default_icon": "browseraction.png",
          "default_title": "Sample",
          "popup": "popup.html"
      },
      "content_scripts": [
          {
              "matches": [ "&lt;all_urls&gt;" ],
              "js": ["scripts/contentscript.js"],
              "run_at": "document_end"
          }
      ]
  }
  
  contentscript.js
  ===================
  var elems = document.getElementsByClassName("header");
  alert( elems.length );
  </code></pre>