Unlike client{X,Y} which is relative to the current viewport, these
offsets are relative to the left edge of the document (i.e they take
scroll offset into account).
The intent is to use these to autogenerate prototype declarations for
Window and WorkerGlobalScope classes.
And the spec links are just nice to have :^)
These provide the cursor coordinate within the viewport at which the
event occurred (as opposed to the page relative coordinates exposed via
offsetX, offsetY).
