There are several major browsers that provide support for HTML5 video codec formats. However, this support is not universal and similar across all browsers. It is likely that you will have to produce your content in three different codec formats in order to cover all of the modern browsers. 

This situation may change as more and more companies align behind one or two formats. Here is matrix of browsers and commonly supported codec formats:

• Firefox supports: WebM, Ogg Theora
• Opera  supports: WebM, Ogg Theora
• Safari supports: MPEG-4 H.264
• Chrome supports: WebM, Ogg Theora and MPEG-4 H.264
• IE supports: MPEG-4 H.264  

As you can see, in order for your application that has embedded video or audio files to work, you need to encode it all three formats: WebM, Ogg Theora and MPEG-4 H.264. There are other formats that HTML5 relies on but these three are open source and widely used as of right now.

Examples of how to code it in HTML presented below:

<video src="TestVideo.ogv"></video>

<video src="TestVideo.webm"></video>

<video src="TestVideo.mp4"></video>