[whatwg] Add 'type' attribute to <mark>

[Pentasis]

I hope I am doing this right, I am not used to mailing lists ;-)

Anyway, following some discussions on the web regarding footnotes/side notes 
I have found that there is a need for some form of element to mark these up. 
The most commonly accepted element at the moment seems to be to use the 
<small> element. But this is clearly a wrong use of semantics.
As the <mark> element has different usages defined on it already why not 
include a "type" attribute (or similar) that defines what it is used for. 
One of these types would then be "footnote", others could be (relating to 
what is already in the spec) "term", "highlight" etc. (I am sure others 
would be much better at thinking up names than I am).
Esp. in light of the fact that the spec states that UA will probably have to 
provide cross-linking would make this an ideal element for footnotes/side 
notes.

Bert