Easy solution, don't leave random webgames running when you're done with them, and if it doesn't noticeably impact performance...?

You don't need a game for that, any website will do, specially with background workers.

Exactly!

Who's checking for this stuff on every blog page open, actually? I wish I was, but, usually am not.

Modern browsers will flag and slow down tabs that abuse the cpu. You could probably still mine but it’s a slow drip.