Problem is, adding to the bl… sorry, richness, doesn't remove the old cruft.
How about having an application platform that would be just an application platform? No HTML, no CSS, no built in multimedia. Just a VM, a viewport, audio, and inputs (and local storage if the user allows it).
We already tried that - Java Applets. Silverligt was the same idea. And Flash I guess? All attempts failed in the marketplace. It turns out using higher level standards like HTML, CSS, URL's etc actually provide a a lot of value.
Obligatory point that Flash didn't fail in the marketplace. Rather, it was wildly popular, so much so that Microsoft eventually felt the need to develop a comparable tech -- Silverlight.
Flash on the web faltered rapidly after many years of efforts by Mozilla, Apple, Opera, and associated individuals. They were looking to move the web 'forward' had a high-profile disagreement with W3C, so they started their own standard-setting collaboration to specify HTML5 and associated JS APIs. The blogosphere eagerly awaited the results, which promised to formally bring multimedia and rich interactivity to HTML, without having to use a vendor plugin.
When Apple announced that Flash won't be supported on the upcoming first iPhone, it was over. After a few years, when apps came to the iPhone, Adobe failed at marketing the fact that their Flash assets can be compiled into iPhone apps using Adobe AIR.
With existing Flash assets effectively relegated to desktop-only, it was only a matter of time before it was pushed out of the standard browser stack. Although later, both Microsoft and Google shipped Adobe's plugin (with better process isolation) together with the browser or the OS, and hooked into their respective auto-updaters, Flash was on its way out.
There is an open source version Gnash. I installed it on a laptop a few years back and it ran fairly well, but a bit slow compared to the real Flash. Why do you think that didn't take off?
They didn't fail in the market, they were actually quite popular (maybe not java, but flash definitely). It is just that Apple killed them by not making them run on mobile. That's more of a case of a major player strong forcing the market.
I'm all for building from scratch a lean minimal runtime platform as you describe (that's what operating systems should be), also it should be "easy", take a minimal Linux kernel and add some thin APIs on top. But good luck turning this into a project that gains any traction.
Using the browser platform to bootstrap such a thing isn't such a bad idea in comparison... sooner than later we need to depreciate the ugly parts, like WebRTC, WebAudio and trim some fat here and there, but other platforms have their ugly parts as well (look at the mess that is Android).
You need a browser in the first place. The standards are now so big that to write a modern, compliant browser, you need to be an international corporation.
A market with only 4 competitors (Microsoft, Google, Mozilla, Apple), is not a market. It's an oligarchy.
How about having an application platform that would be just an application platform? No HTML, no CSS, no built in multimedia. Just a VM, a viewport, audio, and inputs (and local storage if the user allows it).