Baselines (was RE: Units, font sizing, and zoom suggestion for CSS 3)

Baselines
=========

(This is just an side remark message to my two parallel messages
on "font-size" and "em".)

The following baselines should be maintained for all text:

high baseline -- Used for drop caps, and for alignment of Devanagari and
  similar scripts. If no high baseline is declared by the
font,
  an approximation can for example be computed from the low 
  baseline and the ascent of the bounding box for capital H
  (or similar) over the low baseline, or the ascent of a
  "squarish" ideograph over the central baseline.

central baseline -- Exactly(?) halfway between the low baseline and the
  high baseline. Used for aligning ideographs, kana, and
Hangul
  syllables (probably for Ogham too).

low baseline -- Used for alignment of Latin, Greek, Cyrillic, Hebrew, and
  Arabic (and probably many other scripts too).

math baseline -- Used for aligning math expressions.  Should be at the
  center height of operators like +, =, <, and similar.  The
math
  baseline can be at about half ex height, or more often at
half
  cap height.  If a math basline is missing, use the central
  baseline as an approximation.

The distance between the high baseline and the low baseline is 
(should be) the cap height.

The central baselines of successive lines of text is separated by
the line-height property value.

Inline objects must be aligned according to the preferred alignment for
the inlined object.  E.g. an inline math expression should be aligned 
according to the math baseline (so the math baseline must be maintaned
for the surrounding, 'ordinary'm text).

  Kind regards
  (still trying to get this "spec.d/done right")
  /kent k