HTML5 is the fifth revision and newest version of the HTML standard. It offers new features that provide not only rich media support, but also enhance support for creating web applications that can interact with the user, his/her local data, and servers, more easily and effectively than was possible previously.
Because HTML5 is still being developed, changes to the specifications are inevitable. Therefore, not all of its features are supported by all browsers yet. However, Gecko, and by extension, Firefox, has very good initial support for HTML5, and work continues toward supporting more of its features. Gecko began supporting some HTML5 features in version 1.8.1. You can find a list of all of the HTML5 features that Gecko currently supports on the main HTML5 page. For detailed information about multiple browsers' support of HTML5 features, refer to the CanIUse website.