[css3-fonts] proposal for 'font-size-adjust: auto'

I'd like to propose an additional value for the 'font-size-adjust'
property, in addition to the current 'none' and <number> values:

  'auto'
     Behaves just like <number>, except the number to use is the
     aspect value of the first font in the initial value of the
     'font-family' property.  Instead of the first font, user agents
     may use a different font in that list (the initial value of
     'font-family') based on what character ranges the fonts in the
     list provide and the language of the document or content.

This 'auto' value has two major use cases that I can see:

 (1) It works better than <number> for authors who want the evening
 out of x-heights between fonts (particularly monospace and
 non-monospace) but want to use sizes that are relative to the
 user's default font size.  Using <number> essentially multiplies
 the user's default font size by an unknown adjustment factor, which
 'auto' does not.

 (2) It can be useful as an initial value for user agents,
 particularly as a replacement for the current hacks through which
 some user agents use a different default font size for monospace
 and non-monospace fonts.

I think it may even make sense to change the spec such that 'auto'
is an allowed initial value for 'font-size-adjust' (in addition to
'none').

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/