[dom] Document.defaultCharset

foolip has just created a new issue for https://github.com/whatwg/dom:

== Document.defaultCharset ==
IE, Blink and WebKit all have `Document.defaultCharset` and the usage 
in Chrome isn't low enough that it can be trivially removed:
https://www.chromestatus.com/metrics/feature/timeline/popularity/428

Can we specify this? The concept itself is already referenced as "an 
implementation-defined or user-specified default character encoding" 
in HTML's [determining the character 
encoding](https://html.spec.whatwg.org/#determining-the-character-encoding),
 and assuming the side effects of that are observable, this wouldn't 
actually expose new information about users.

This should probably be blocked on 
[Document.charset](https://www.w3.org/Bugs/Public/show_bug.cgi?id=27436).

See https://github.com/whatwg/dom/issues/58