title: Can you use both the async and defer attributes on a HTML tag?
body: <p>I would like to load in the following JavaScript code using both <code>defer</code> and <code>async</code>:</p>
  
  <p><code>&lt;script defer async src="/js/somescript.js"&gt;&lt;/script&gt;</code></p>
  
  <p>Since <code>defer</code> is supported by Internet Explorer 5.5+, as you can see at <a href="http://caniuse.com/#search=defer" rel="noreferrer">CanIUse.com</a>, I would like to gracefully fallback to using defer if async is not available. Async I think is better to use when it is available but its not supported until Internet Explorer 10.</p>
  
  <p>My question is therefore is the above code valid HTML? If not, is it possible to create this situation using JavaScript of gracefully falling back to using <code>defer</code> on a script when <code>async</code> is not available?</p>