A list of some of the most frequently asked questions
window.onerroron your page, and reports any findings from there. Though this is non-standard (like Ajax once was), it is supported by all popular browsers. Of course things are a little more complicated than that, but that's the gist.
window.onerrorare supported by Errorception. Currently, this means that Internet Explorer, Firefox, Chrome 10+, Safari 4+, Opera 9+, iOS5+, and Chrome on Android are supported. That's practically every browser out there!