> Browsers already have to know what the mappings are in order to support them, they just don't share that information with users.
Unfortunately the way this works in the DOM makes that undecidable. An arbitrary piece of JS code from the web page gets invoked for every keystroke, if the page code has added a handler for key events. If it has not called cancelDefault() on the event object by the time it returns, the browser takes the default action. The question of whether or not user JS code will invoke cancelDefault() for a given event is equivalent to the halting problem.
Unfortunately the way this works in the DOM makes that undecidable. An arbitrary piece of JS code from the web page gets invoked for every keystroke, if the page code has added a handler for key events. If it has not called cancelDefault() on the event object by the time it returns, the browser takes the default action. The question of whether or not user JS code will invoke cancelDefault() for a given event is equivalent to the halting problem.