The Content-Type header tells your Firefox 3 browser whether to use the HTML rendering engine, the XHTML rendering engine, the XUL rendering engine, or the Adobe Flash rendering engine. Is that what you mean?
No, that does not solve the problem of the Google monopoly. Not a fixed set contained within one browser engine, but the browser engine itself, like Webkit vs some other engine vs some specialized engines. If your engine-host browser does not have a give engine or specific version of an engine it downloads it and caches it. WASM/Wayland makes the idea feasible, loading lightweight WASM modules.