I think a decent layout or CSS library would be useful outside of a web browser, too.
Then I would also only focus on the subset of websites that are "documents", not "apps". If I could decide, HTML6 would have two profiles: one ultra restricted (maybe no legacy stuff and no cross site scripting) for "documents", and one where you can do all kinds of crazy stuff like "web USB" for "apps". That's not going to happen because Google and Apple like the "open" web as complex and messy as it is, because it gives them total control as you know. But it doesn't stop a browser vendor from building a browser with two engines - your own engine for the majority of documents and chromium for webapps.