Force HTML5 youtube video

I've found the solution :

You have to add the html5=1 in the src attribute of the iframe :

<iframe src="http://www.youtube.com/embed/dP15zlyra3c?html5=1"></iframe>

The video will be displayed as HTML5 if available, or fallback into flash player.

Whether or not YouTube videos play in HTML5 format depends on the setting at https://www.youtube.com/html5, per browser. Chrome prefers HTML5 playback automatically, but even the latest Firefox and Internet Explorer still use Flash if it is installed on the machine.

The parameter html5=1 does not do anything (anymore) now. (Note it is not even listed at https://developers.google.com/youtube/player_parameters.)