YouTube URL in Video Tag

The most straight forward answer to this question is: You can't.

Youtube doesn't output their video's in the right format, thus they can't be embedded in a
<video/> element.

There are a few solutions posted using javascript, but don't trust on those, they all need a fallback, and won't work cross-browser.

Video tag supports only video formats (like mp4 etc). Youtube does not expose its raw video files - it only exposes the unique id of the video. Since that id does not correspond to the actual file, video tag cannot be used.

If you do get hold of the actual source file using one of the youtube download sites or soft wares, you will be able to use the video tag. But even then, the url of the actual source will cease to work after a set time. So your video also will work only till then.

MediaElement YouTube API example

Wraps the YouTube API in an HTML5 Media API wrapper, so that it can be programmed against as if it was true HTML5 <video>.

<script src="jquery.js"></script>
<script src="mediaelement-and-player.min.js"></script>
<link rel="stylesheet" href="mediaelementplayer.css" />

<video width="640" height="360" id="player1" preload="none">
    <source type="video/youtube" src="http://www.youtube.com/watch?v=nOEw9iiopwI" />
</video>

<script>
    var player = new MediaElementPlayer('#player1');
</script>

This would be easy to do :

<iframe width="420" height="345"
src="http://www.youtube.com/embed/XGSy3_Czz8k">
</iframe>

Is just an example.