Inconsistency in getAttribute and setAttribute regarding invalid attribute names

[DemiMarie]

Currently,

<div #x=y></div>

parses successfully. The #x attribute of the resulting DOM node can be successfully queried using getAttribute — but trying to modify it with setAttribute throws.

It may not be possible to change this, but it is still a poor API, and such documents have no serialization as XML. Perhaps a deprecation warning could be logged when getAttribute is called with such a name?

[domenic]

Fixing setAttribute is tracked by whatwg/dom#449, which is a bit stalled. Since setAttribute is defined in DOM, let's use that issue tracker instead of this one.