The WHATWG Web Applications 1.0 specification is an initiative by browser vendors (Opera and Mozilla seems to be included) and other parties to create the next generation of web application XHTML.

Notable items are: the context menus, a direct-mode graphics canvas, inline popup windows, and server-sent
events.

The latter is especially interesting for load/performance considerations: fundamentally all web applications have to maintain a channel to the server to learn about events that are important for the user (think of a chat app, for example). At the moment this is done by the client (browser) periodically polling the server, which is a recurring load for the server (it happens even if you are not doing anything). The server-sent events technology promises to turn this into server-push: the clients will not have to poll, and the server will rather push the information to clients that are interested.

Opera seems to be the first to implement the technology in the 9.0 browser. It seems Opera is working hard to become the darling of web applications developers: first their rendering and editing improvements, and now this starts to make Opera a very compelling platform for web apps.