Could we just support both? It really depends on how you read it, but
onkeyup and keyup should be interchangeable. I guess implementations
could add this as an extension (to avoid such typos).

I'd originally argued for supporting the "on" version of the names, but the argument that carried the day was that the actual name of the event would be both more concise, and more accurate.

The idea of supporting both is interesting (be liberal in what you accept, after all), I don't think that it would cause any name conflicts.

However, at least in this case, the documentation is quite clear on this point:

The DOM event name is the actual DOM event name (for example: "click") as opposed to (for example: "onclick").

I suspect, however, it may be better to wait and see if this is a pain point for users...  It'd be easy to add this later without endangering existing code.