title: getPropertyValue("backgroundColor") returns an empty string
body: <p>This is my problem:</p>
  
  <pre><code>var mycss = window.getComputedStyle(myelement);
  </code></pre>
  
  <p>returns a CSSStyleDeclaration object:</p>
  
  <pre><code>CSSStyleDeclaration {0: "animation-delay",..., backgroundColor: "rgb(0, 0, 0)",...}
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/MH4d7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MH4d7.png" alt="console.log(mycss);"></a></p>
  
  <p>Then, I want to get the background color, but</p>
  
  <pre><code>mycss.getPropertyValue("backgroundColor");
  </code></pre>
  
  <p>returns an empty string <code>""</code>!</p>
  
  <p>Why??</p>