My theory is that Mozilla and Google are in a race to develop the most advanced html5 engines because they know that if a killer html game or app gets developed in their browser, everyone who wants to use the app will switch to them while the other implementations tries to catch up.
It will also prevent new browsers from trying to pop up, unless they simply fork and try to keep up with one of the two major engines. Google and Mozilla know they can outpace any competitors that are trying to innovate in the web space by having the "latest" html5 features integrated. In example, unless Microsoft pours a bunch of money into it, or Internet Explorer forks Mozilla or Google, they can pretty much count that competitor out.
Is this supporting an open web?
What's the problem with focusing on open source plugin virtual machines that can work in every browser regardless of that browser's version?