[CSS21] rule on % height in auto-height container should apply to used value, not computed value

http://www.w3.org/TR/CSS21/visudet.html#propdef-height says, in the
definition of <percentage>:
  # If the height of the containing block is not specified
  # explicitly (i.e., it depends on content height), and this
  # element is not absolutely positioned, the value computes to
  # 'auto'.

I believe this statement (and the corresponding bit of the "Computed
value" line) should be updated to reflect that this change happens
at the *used value* level but not at the *computed value* level.  I
think it is part of the principle of the computed vs. used value
distinction that computed values of properties should not depend on
the identity of or properties of the element's containing block,
though I don't know if this principle was previously stated.  Thus,
I think it should have been changed when we introduced the concept
of used value.

(I'd note that the prose for max-height and min-height is different,
but should probably be the same, once this is corrected.)

A testcase is attached.  Gecko and WebKit agree with my
interpretation; Opera does not.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                           http://www.mozilla.org/   𝄂