I wish to be able to browse websites, which contain (a) text, (b) associated images, (c) low forms of interactivity, say, comments, or interactive graphics.

This is pretty much the web 2012, or most of German-language web today still.

For browsergames, let’s just use the same paradigm as with apps instead: Bundle them via node+webkit, and integrate a "run locally" API into browsers that automatically downloads a program and runs it locally in a sandbox, but seperately.

In the long term, for games we’ll need to develop a different concept anyway.

But there’s a good reason why no one develops 3D games in PDF, despite PDF supporting 3D objects, scripting, and modification of the document via scripts.