HTML5, features of which initially began arriving in Gecko 1.8.1, is the newest version of the HTML standard. It offers new features to provide not only rich media support but also enhanced support for creating web applications that can interact with the user, his or her local data, and servers more easily and effectively.
Because HTML5 is still in the design stage, changes to the specification are inevitable. Because of this, not all of its features are supported yet by any browser. However, Gecko (and by extension, Firefox) has very good initial support for HTML5, and work continues toward supporting more and more of its features. You can find a list of the HTML5 features Gecko supports on the main HTML5 page.
The HTML5 doctype
The doctype for HTML5 is very simple. To indicate that your HTML content should be interpreted as HTML5, simply use:
This very simple doctype will cause even browsers that don't presently support HTML5 to enter into standards mode, which means they'll interpret the long-established parts of HTML in an HTML5-compliant way while ignoring the new features of HTML5 they don't support.