title: Fallback picture for <PICTURE>
body: <p>Is <code>&lt;PICTURE&gt;</code> officially in HTML5?  I can't find it in w3schools.com.</p>
  
  <p>Assuming it is official, is the source attribute of the fallback <code>&lt;img&gt;</code> element <code>src</code> or <code>srcset</code>.  I am seeing some web sites using <code>srcset</code> and they don't work in any version of IE, but <code>src</code> works in IE. </p>