title: Putting -moz-available and -webkit-fill-available in one width (CSS property)
body: <p>I want to make the width of the footer browser-independent.</p>
  <p>For Mozilla I want to use the value of <code>-moz-available</code>, and when a user uses <a href="https://en.wikipedia.org/wiki/Opera_%28web_browser%29" rel="noreferrer">Opera</a>, then CSS should get the values from <code>-webkit-fill-available</code>.</p>
  <p>How can I do this in CSS 3?</p>
  <p>I tried to do something like this:</p>
  <pre class="lang-css prettyprint-override"><code>width: -moz-available, -webkit-fill-available;
  </code></pre>
  <p>This won't give the desired results.</p>