User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.65 Safari/537.36 Steps to reproduce: Versions: 23.0.1, 26.0a1 (2013-09-12) Visiting a video directly in the browser plays it fine. However, when done through a <video> element or javascript, video will not play. Steps (in console): document.createElement('video').canPlayType('video/mp4'); Steps (in html): <video autoplay> <source src="http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4" type="video/mp4"> </video> Actual results: javascript: empty string ("") html: [17:40:07.790] Specified "type" attribute of "video/mp4" is not supported. Load of media resource http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4 failed. @ file:///Users/kastner/Desktop/video.html Expected results: javascript: "maybe" html: a beautiful video playing

WFM with FF26 on Win 7.

I can reproduce using that URL on Firefox 23.0.1 on OS X 10.8.4

Confirmed in 27.0a1 2013-10-06 mac os x 10.7.5

This is pending implementation of mp4 playback on Mac. It works on Win7, and on Linux if the appropriate gstreamer plugins are installed. You can follow bug 799318 for overall status tracking.

I'm not sure why it would work when 'visiting directly.' Maybe you're hitting the Quicktime plugin in that case? That won't work with the html <video> element; a the page would have to provide an <object> fallback itself.