[css-flexbox] concepts of baselines

http://dev.w3.org/csswg/css-flexbox/#flex-baselines has a bunch of
text about how CSS 2.1 doesn't define the baseline of block and
table boxes, etc.  However, it misses a distinction present in the
underlying concepts.

If CSS 2.1 defined a baseline, it would need to define *two*
concepts of baseline, a last-line baseline concept used for
inline-block and inline-table baseline alignment in
http://www.w3.org/TR/CSS21/visudet.html#propdef-vertical-align
(which also uses the bottom edge for overflow != visible, and which,
for compatibility, uses the first row of a table), and a first-line
baseline concept used for table cell baseline alignment in
http://www.w3.org/TR/CSS21/tables.html#height-layout .

-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)