Choosing the Set of Renderable Strings

[James Kass via Unicode]

Display behaviour which is script-specific should be handled by the
rendering/shaping engine.  Only that which is font-specific should be
handled by the font.

The font's OpenType tables will include pointers to presentation forms
which aren't directly encoded, the location and repertoire of which
would naturally differ from font to font.  Likewise, the font's GPOS
tables will handle things such as mark positioning, because each
font's metrics are going to be different.

Because the USE apparently accesses current on-line Unicode data, the
USE will re-order anything which needs to be moved around.