Where previously we might have used Flash or other technologies to convey audio or, especially, video content to users, HTML5 offers powerful - yet simple - ways to embed media content into HTML documents.
There has been a lot of discussion about what file formats should be recommended in the HTML5 specification, but it is currently file format neutral. At one point the specification recommended OGG, but it removed that recommendation, in part due to Apple's refusal to implement OGG.
From a practical point of view, to be sure that your media will play in your user's HTML5-compliant browser:
Note that you must configure your web server to deliver the audio and video mime types that you use.
In Apache, you can use AddType video/ogg .ogv .ogg. For more information, see https://developer.mozilla.org/en/Properly_Configuring_Server_MIME_Types and scroll down to the section labeled "How to set up your server to send the correct MIME types."
In IIS, you can add Mime types through Computer Management. For more information, see http://technet.microsoft.com/en-us/library/cc725608(WS.10).aspx.
Your web server must be configured to serve audio and video files with the proper mime type.
This will not be an issue for this class if you are opening the files locally.