title: Why does inner DIV spill out of outer DIV?
body: <p>I have been long away from HTML and CSS, can't find the solution to this simple problem.</p>
  <p>I have one div inside the other. Outer Black and inner orange.</p>
  <p><a href="https://i.stack.imgur.com/hsZkE.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hsZkE.gif" alt="enter image description here" /></a></p>
  <p>My HTML and CSS is :</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>#outer {
    position: fixed;
    width: 30%;
    height: 30%;
    background-color: black;
  }
  #inner {
    width: 100%;
    height: 100%;
    margin: 5px;
    background-color: orange;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div id="outer"&gt;
    &lt;div id="inner"&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  <p>Why is my inner DIV spilling out of the outer ? How do I fix it without giving fixed dimensions?</p>