Sandboxing is hard, very hard. You would be afraid to even open a browser window if you know enough. If you like to have a strong sandbox, you cannot use plugin

Sandboxing is hard, very hard. You would be afraid to even open a browser window if you know enough. If you like to have a strong sandbox, you cannot use plugins. You cannot even use fonts or render images - not to speak of video. Using fancy HTML5 APIs like to access file system or device sensors? Gone. WebGL, well... But this is the case right now with "javascript as the assembly of the web". So, nothing changes in the polyglot browser VM except that non-javascript languages can target it directly rather than go through javascript.