title: Get element's custom css properties (-mystyle) using JavaScript
body: <p>In an application where certain elements have custom CSS properties, is there any way to retrieve such a value via JavaScript?</p>
  
  <p>e.g.</p>
  
  <pre><code>&lt;div id="myDiv" style="color:#f00;-my-custom-property:upsidedown;" /&gt;
  </code></pre>
  
  <p>I can access the color attribute via these two methods:</p>
  
  <pre><code>document.getElementById('myDiv').style.getPropertyValue("color")
  document.getElementById('myDiv').style.color
  </code></pre>
  
  <p>But these do not work for custom properties. Is this supported at all?</p>