I'm not sure what you're talking about.

1. As we're seeing here, browser developers determine what content the browser will parse and process. This happens in both directions: tons of what is now common JS/CSS shipped first as browser-specific behavior that was then standardized, and also browsers have dropped support for gopher, for SSLv2, and Flash, among other things.

2. Browsers often explicitly provide a transformation point where users can modify content. Ad blockers work specifically because the browser is not a "servant" of whatever the server returns.

3. Plenty of content can be hosted on servers but not understood or rendered by browsers. I joked about Opera elsewhere on the thread, which notably included a torrent client, but Chrome/Firefox/Safari did not: torrent files served by the server weren't run in those browsers.