[css-sizing] block intrinsic width accounts for floats incorrectly

http://dev.w3.org/csswg/css-sizing/#block-intrinsic says:
  # The max-content measure of a block container box is the measure
  # of the box after layout, if all children are sized under a
  # max-content constraint. 

I don't think this is what any current implementation does (although
some implementations -- old Gecko and old IE -- used to compute it
that way), and it's distinguishable from implementation behavior in
many testcases.  Implementations actually use heuristics (which I
believe match pretty well, but not perfectly) to determine when
float intrinsic widths should be added to block intrinsic widths and
when they should just be max()'d.  I believe these heuristics should
be specified.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)