Here are some references for YouTube, iframe and HTML5.
The format used to encode the video determine if the video will play with the HTML5 player. The viewers’ browser is a factor as well.
Moving to embeds
Given the progress we’ve made with HTML5 , we’re now defaulting to the HTML5 player on the web. We’re also deprecating the “old style” of Flash object embeds and our Flash API. We encourage all embedders to use the iframe API, which can intelligently use whichever technology the client supports.
YouTube HTML5 Video Player
How To View HTML5 Videos on YouTube