[css-fonts] Stylistic alternates

I don't understand how these are supposed to work in the current draft.

At the font level, there's a "salt" feature, implemented by a type 3
(alternate substitution) lookup. For each glyph id, this will result in an
ordered list of alternate glyph ids.  The usual behavior in an editing
environment is that the UI allows the user manually to select the
appropriate alternate.

An important point is that alternate N for glyph X does not necessarily
have any stylistic relationship to alternate N for glyph Y: the effect of a
particular alternate index is glyph-specific. For example, in the Microsoft
Consolas font:

- alternate #1 for "f" adds a horizontal base
- alternate #1 for "i" makes the bottom of the vertical stroke curve to the
right (the default glyph has a horizontal base)
- alternate #2 for "i" curves the top of the vertical stroke to the right
- alternative #1 for "j" is like alternate #2 for "i"
- alternate #1 for "l" is like alternate #1 for "i"
- alternate #1 for "r" is like alternate #1 for "f"
- alternate #2 for "r" changes the curvature of the horizontal part of the
glyph
- alternate #3 for "r" is like alternate #1 for "j"
etc

So the kind of thing I would want to do is to define stylistic feature
value names such as:

- horizontal-base: use alternate #1 for "f" and  #1 for "r"
- curve-base: use alternate #2 for "i", #1 for "j" and #3 for "r"

If I have understood the current draft correctly, it only allows the
definition of a stylistic feature value name by using a particular
alternate index uniformly for all glyphs. This doesn't seem to me to be
quite what is needed.

James