&gt; [...] the meaning that it communicates isn&#x27;t feature data.<p>But it kind of is (to some degree, at least). VP9 was recently added to Firefox 28. If the User Agent says the browser is Firefox 27, then you probably shouldn&#x27;t try serving VP9 to that browser.<p>Obviously the User Agent doesn&#x27;t communicate the complete feature set. But some useful inferences regarding features can be derived from it.