The theory is that if all the User-Agent providers implement all these algorithms exactly as specified, complete interoperability will be achieved and people who build Web applications need no longer concern themselves with the differences between User Agents. Which would of course be wonderful.
Will it work? Nobody knows; it’s a science experiment. Just because nobody has ever succeeded in specifying a workable networked object model doesn’t mean this project will likewise fail. But it does mean that when considering the future of HTML5, we should recognize that this is a very hard problem, and there’s no guarantee that that part of it will come off.
From a blog post on HTML5 by Tim Bray.