H.264 is a codec that is commonly used in embedded media and hmtl5 enabled browsers can open the file directly.
Youtube video codecs
Firefox 4 and above along with the webkit based browsers will play. Firefox 9 had html5 improvements and works very well using the WebM codec. Webkit based browsers use h.264 and of those xxxterm and epiphany are stable (OpenBSD 5.1 current) while midori often core dumps.