Same visual quality/performance as a native game is probably not possible and every game in js will perform worse AND create more load (and waste energy).
Why is everyone trying to move games and big applications to the web?
However I can see exactly why people are doing this. There have been a big explosion of indie games recently and that can be put down to iOS and Steam. Both make it super easy to get games.
Anything that reduces friction is going to be very appealing to game makers. And installing Steam is friction. If playing your game simply involves visiting a link - thats pretty easy. As HTML5 matures, as more people use modern browsers, and as Flash becomes less reliably installed on browsers, expect to see a lot of casual games moving to HTML5.
I am a gamer with an interest in the indie sector but don't have steam because my first experience with that extra platform abstraction - Origin - has put me off. Neither do I want a game as a web page. I want a gaming experience and that for me is still a standalone executable. I even dislike not having a physical copy of digital distributions.
There are plenty of game types -- some "casual", some not -- that are perfectly satisfied by either lower hardware or arcane/wasteful software stacks. And you can benefit from the ease of development, distribution and cross-platform capabilities. Energy, sure, but with browser-based 2D games that's mostly CPU, so given the identical native game we're talking about what, a 30W difference? Might as well tell people not to play triple-A games with their SLI GPUs...
This isn't really a new discussion, as you can bring the same argument for desktop apps, too. Look at something like extjs, compared to native GUIs. Wastes heckuva lot of resources to try to look like a real application and will never reach that (like early Java...). But it's darn convenient...
Full 3d is a different problem.
However, a game like Monaco, with super simple controls but really interesting game play, could absolutely be built in the browser. This gives an easier distribution medium, more control over who is using the game, and if you're already a webdev it fits in nicely with your pre-existing skill set.
The bar is significantly lowered for a lot of weekend warrior game makers if you're going the 2d route.
It's not there yet for all types of games or applications, for a number of reasons, but the performance aspect of it is gradually coming under control and most of the issue now rests on API functionality and browser compatibility.
Why not compile everything to brainfuck? It reminds me of that post of the guy who booted linux on an 8 bit uC :D