Here you go.
https://davidben.net/event-test.html

There are two tests in there. The first tests `addEventListener` and `removeEventListener` being called mid-dispatch. It's identical to the one in the commit and seems to be consistent between browsers.

The second is the test of `onfoo` and `addEventListener` interaction from the HTML spec. You hit the Run button to trigger it. The spec behavior is ONE TWO THREE FOUR, but WebKit doesn't match. To implement the spec behavior (or the WebKit behavior), we need custom setters for all the supported onfoo attributes so you can react to the first time they're set, maybe with a `REventTarget.registerAttribute` function. (Which is consistent with the real `EventTarget`; if you emit a custom event, it doesn't actually do the `onfoobar` thing. Only the built-in ones which have them defined.)