> When I reset my browser (and IP address), I expect a clean slate, and with this API I won't get it.
Browsers don't currently give you that, and never have. You can download files, and these files remain on your system even if you re-install your browser.
> This may sound dumb too, but I expect my browser just show me web.
Browsers are just a sandbox for running applications that are downloaded over the network. Some applications need web cam access, some need access to files on your system, some need geolocation, and so on. Browsers let you choose what permissions to give applications. Because of this, you're completely free to let your browser just "show you the web" in whatever sense you like.