title: Child div should cover whole area of its parent div
body: <p>Below is my code</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.not-my-div {
    width: 300px;
    height: 200px;
    background-color: antiquewhite;
    border: 1px solid black;
  }
  
  .my-div {
    float: left;
    width: 100%;
    height: 100%;
    background-color: aqua;
    opacity: .5;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="not-my-div"&gt;
    &lt;p&gt;Some Text Here&lt;/p&gt;
    &lt;div class="my-div"&gt;&lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I want my-div should cover whole area of not-my-div.</p>
  
  <p>not-my-div is not in my control so I can't do any thing to it and its
  width and height is not fixed.</p>
  
  <p>Below is sample what I want.</p>
  
  <p><a href="https://i.stack.imgur.com/ji7RG.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ji7RG.jpg" alt="enter image description here"></a></p>