HTML5 is a new platform for web development that includes features like offline application caching, 2D drawing with Canvas, local storage, web workers, notifications, and web sockets. Python can be used in the HTML5/RIA space through frameworks like Pyjamas that compile Python to JavaScript. However, for client-side development JavaScript will likely continue growing in popularity over Python due to its seamless integration and the lack of compelling reasons to replace JavaScript with Python in browsers.