title: getElementByClassName always returns empty array
body: <p>I´m trying to get elements by class name out of my Google Chrome Extension, but I´m getting always an empty array back.</p>
  
  <p>The classes I´m looking for do definately exist.</p>
  
  <p><img src="https://i.stack.imgur.com/akBTA.png" alt="enter image description here"></p>
  
  <pre><code>var myClasses= document.getElementsByClassName("className");
  </code></pre>
  
  <p>I´m using the Google Chrome Browser.</p>
  
  <p>Am I missing something?</p>