I've seen custom contextmenus generated on pages[0] that override the built-in context menu but for some reason I've yet to come across a page[1] that adds to the existing context menu until today.
>but for some reason I've yet to come across a page that adds to the existing context menu until today.
the reason is likely because browser compatibility for the context menu js api is pretty minimal [1]. firefox can use it on pages that are only going to be displayed in firefox, but for the internet at large it's pretty useless.
What about Google Docs? Their context menu seems to work cross browser (even on ie9!). Maybe my assumption that they're using HTML5 for this is wrong but if I'm right then what you see on Gdocs is a complete replacement of the regular menu which is pretty cool but also quite annoying for me at times. Please correct me if I'm wrong (which is very likely).
Google docs is not appending stuff to the browser's context menu. they're catching the right-click action and replacing the standard context menu with their own one drawn in HTML.
