Sufficiency/checkpoint satisfaction criteria (repost of last week's draft)

As agreed at the meeting, I am here posting, for the second time, my
draft of sufficiency/checkpoint satisfaction? criteria for working
group review. I have changed some of the commentary (and the title) to
reflect discussion at the meeting.

Please note: at the meeting it was decided to incorporate these,
albeit perhaps after some editing, into the next internal working
draft of WCAG 2.0.

Review is requested, particularly in relation to those criteria which
state, explicitly, that yet to be defined complexity thresholds must
be met.