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.
