Surely if shaping, presentation form etc information is to be encoded in a font or rendering mechanism at all, it must be script-specific not language-specific. That is one reason why there are quite a number of Persian, Urdu etc variant characters in the Arabic script block - they cannot be unified with the otherwise identical Arabic characters because they have different shaping behaviour.
Both kinds of information may be necessary, depending on the writing system (particular application of a particular script to a particular language). Encoding a particular glyph as U+00431 in a font cmap table is script-specific information; a glyph substitution lookup that replaces that glyph with a different one when the language is Serbian is language-specific information.
Note, however, that not everything one may want to happen in a font is neatly divisible into script and language. There may be distinct typographic traditions in the setting of the same language. Catering for these in architectures that are nominally limited to script and language distinctions is very tricky.
John Hudson
Tiro Typeworks www.tiro.com Vancouver, BC [EMAIL PROTECTED]
Theory set out to produce texts that could not be processed successfully
by the commonsensical assumptions that ordinary language puts into play.
There are texts of theory that resist meaning so powerfully ... that the
very process of failing to comprehend the text is part of what it has to offer
- Lentricchia & Mclaughlin, _Critical terms for literary study_
