If you're capturing the controls correctly for the major browsers then tab should be fine, it's just not something I'd think of trying :(, it doesn't work in most web games (because you have to disable the built-in browser controls). Same with RMB - it's annoying when web games break the browser so they can use RMB, and very few do it well (also: works badly on a lot of laptops, so most devs don't use it any more).
Standard for 'go back/out' is escape - and doesn't have a special meaning in browsers.
Most games I've played have an on-screen button for 'back' or 'exit', so there's no problem. I'd say 9 in 10 games have an onscreen button, whether or not they also have 'esc'.