I agree that websites should not be able to override your keys. I'm often annoyed when I press '/' in firefox, expecting quicksearch to come up, and instead the website hijacks it for their own search.

However, I don't think that this should be solved at a website level. Websites shouldn't know or care what keys do in your browser; they should simply provide reasonable shortcut keys. Browsers should make it so that their shortcut keys cannot be overridden without your explicit permission.