What is the trick to using the html5

I have googled it, and don't find a comprehensive fix for getting the video tag to work in all the browsers. I have the following code as is suggested many places, but still can't get it to work on android, IOS, and Safari. Any ideas? It works find in Chrome and IE.
<video id="myVideo" controls preload="metadata"> <source src="test.mp4"/> <source src="test.mp4" type="video/webm; codecs=vp8,vorbis"/> <source src="test.mp4" type="video/ogg; codecs=theora,vorbis"/> </video>