&gt;  YouTube has to use the User Agent to try to serve videos that the client can actually play.<p>Why? We have JavaScript feature detection for video tags: <a href="http:&#x2F;&#x2F;diveinto.html5doctor.com&#x2F;detect.html#video-formats" rel="nofollow">http:&#x2F;&#x2F;diveinto.html5doctor.com&#x2F;detect.html#video-formats</a>