Perhaps @@unscopable shouldn't be a Set...

[Boris Zbarsky]

On 4/30/14, 2:49 PM, Axel Rauschmayer wrote:
> So Element.prototype is in the variable scope chain of event handlers? Wow. Is this documented somewhere?

You mean other than in the spec?  See 
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#getting-the-current-value-of-the-event-handler 
step 10.

Note that it's not Element.prototype that's in the scope chain but the 
element itself.  It's just that the property lookup on the element will 
walk its proto chain.

-Boris