[CSSOM View] "Document content width / height" doesn't have normative definition

The spec defined the term *document content* as "the area on the canvas
that is rendered upon, excluding content on negative axis" but it seemed to
me that it doesn't normatively define its metrics.

In later sections of the spec *scrollWidth* and *scrollHeight* is defined
using the term *document content width / height*, which is quite vague and
not so instructive for the actual calculation of root element's scrollWidth
/ scrollHeight.

Just have a look at scrollHeight of the root element, according my test
results, Firefox and IE regard it as the margin box height excluding
margin-bottm, Chrome regards it as the padding box height and Opera regards
it as the margin box height.

I think we need some normative definitions here to help user agents
conformantly calculate scrollWidth and scrollHeight of the root element.