So, finally, YouTube has released an updated embed code that support HTML5 videos, and the new style uses <iframe>, "If you use new code, your viewers will be able to view your embedded video in one of our Flash or HTML5 players, depending on their viewing environment and preferences. In instances where HTML5 isn't supported (e.g. our HTML5 player can't play videos with ads), we use Flash." Here's the code:
<iframe class="youtube-player" type="text/html" width="640" height="385" src="http://www.youtube.com/embed/VIDEO_ID" frameborder="0">
"Additional benefit of new style is that it'll eventually allow embeds to work on mobile devices, which typically use a built-in player instead of Flash or HTML5," stated Goole. iPhone, Android phones and other mobile devices use a special app for YouTube, while mobile browsers use plug-ins that detect YouTube's embedding code and replace it with some code that can be handled by built-in YouTube app. New code will allow YouTube to use HTML5 video tag in browsers that support this feature.