Any kind of JavaScript while(true) loop will lock up the page, so you aren't safe from incompetent coders just yet. Depending on the browser and the OS that can

Any kind of JavaScript while(true) loop will lock up the page, so you aren't safe from incompetent coders just yet. Depending on the browser and the OS that can bog down the CPU as badly as Flash. (And there's no move towards parallel execution of in-page dom-touching JavaScript yet - web workers can't do it. Concurrency is such an issue that you can't even pass a function into a web worker in case it has bound variables - it's separate-script-and-message-passing only.)