For almost a week, I've been working with zeno, wisec and others on JavaScript events and HTML Tags; what event can be executed in what tag...
The testing is definitely not finished but I was implementing a JavaScript Unit Testing based test bed for keeping everybody out of clicking on 8700 testcases * nb_browsers...

Anyway, the method I use is to fire a JavaScript event on the load of the document to verify if it works (the information are gathering by the JSUnit Framework).
So, the funny part in firefox is that I can fire almost every event in every tag; you can find an example here where I do something like that:

<acronym onsubmit="alert('TEST')">test</acronym>

The equivalent Internet Explorer version can be find here (it works well... ie does nothing).

I didn't really take the time to think about this but I'm sure something can come from this...

Edit: Wisec found that under firefox you can also fire every events on unexisting tags such as:

<unex ondblclick="alert('TEST')">test</unex >