According to Adam de Boor (a staff software engineer from Google) many upcoming features of the Gmail will be rendered in HTML5. De Boor said that Gmail with HTML5 will be much more efficient. Early tests have proved promising. Google has found that using CSS3 can speed the rendering time by 12 percent.
HTML5 allows building new features for Gmail. For example one feature that the Gmail design team already created is the ability to drag files from the desktop into the browser. However, this feature works only with very few browsers, one of them is Chrome.
HTML5 is currently under development as the next major revision of the HTML standard. Like its immediate predecessors, HTML 4.01 and XHTML 1.1, HTML5 is a standard for structuring and presenting content on the World Wide Web. The new standard incorporates features like video playback and drag-and-drop that have been previously dependent on third-party browser plug-ins such as Adobe Flash, Microsoft Silverlight, and Google Gears. Source: Wikipedia