I want to detect and block certain keyboard shortcuts on a web page. For example, say I want to prevent `alt`+`tab` for switching apps (just an example, assume any global shortcut).

Here's as far as I can think it out:

* attach a keyboard event listener to
`document` (or `window`?)
* use `event.which` to check which key
combination was pressed
* if it was a blacklisted shortcut,
stop the browser from executing it

But, I don't know how to  

**A) detect *multiple keys*** (e.g. `alt` and `tab` together), or  

**B) stop them from executing** (can I just `return false`?).

Can anyone tell me how to accomplish the above?