It shouldn&#x27;t be the only input signal for determining those things, but it can definitely be useful (i.e. &quot;this codec is only supported in vX.Y+ of this browser&quot;, or &quot;vX.Y of this browser has a bug handling this type of video&quot;, etc.).<p>It&#x27;s not like YouTube is a company of full of nothing but dopy idiots. I don&#x27;t think they&#x27;d be using the User Agent if there was a better alternative.