In order to consistently have weight with changing CSS, HTML rendering etc you basically need your own browser - and most of the changes we've seen in the last few years have come out of Google, Mozilla, Microsoft, Apple.
Microsoft need a browser and always will.
You hold the keys to the default search engine for that particular browser, to name an example. Even with the Mozilla-Yahoo deal it's still in MS interest to own a browser so they can push Bing and their other cloud services there.
Note that if other browsers themselves adopt new Windows APIs in competitive response, that actually still benefits Microsoft because ultimately they don't care about the browser for its own sake, they care about increasing Windows API and feature usage across all browsers. But in practice competing browsers tend to be late adopters of new Windows stuff (e.g., even now nothing but IE supports DComp/DManip and Chrome is just very recently getting around to improving its Windows high-DPI and text rendering support), hence Microsoft funds IE.
And this is one area MS hasn't clarified yet, how long will they support IE. It's gonna take time for enterprise to switch to a new platform.
But that's why I love tech.. one day Silverlight (as an example) is flying high, the next it's dead.