Case-sensitivity of Author-defined Identifiers

So, we resolved to make user-defined identifiers (like counter-names and namespace prefixes) case-sensitive to avoid dealing with Unicode case-folding and other complications.

This is fine for things that are entirely user-defined, like counters and namespace prefixes, but Tab noticed that it presents a problem when we get to counter-styles: all our styles right now are predefined keywords, and are thus case-insensitive ASCII. When we use @counter-style to define counter styles, we will be allowing users to create their own counter styles. Will those be case-sensitive? What if the user redefines an existing counter-style?

When UA identifiers (insensitive) and user identifiers (sensitive) co-exist, and UA identifiers are defined in terms of user-defined syntax (e.g. @counter-style), what is the case-sensitivity story?

Various options:

1. @counter-style names are always case-insensitive
   1. via Unicode case-folding
   2. via ASCII case-insensitivity
   3. via Unicode lowercasing
   4. Something else?
2. @counter-style names are always case-sensitive; UA-defined keywords are simply defined for all case permutations. (Overriding one only overrides the permutation given.)
3. @counter-style names are case-sensitive, except those on the UA-defined list are special and are ASCII case-insensitive.

Simon Sapin writes:

I’d prefer to avoid 1A. because Unicode case-folding is very easy to 
implement almost-but-not-quite right, and depends on a locale. (Eg. in 
Python it is tempting to use the 'lower' or 'upper' method of Unicode 
objects, but these only map codepoints one-to-one and are not proper 
case folding.)

Resolved on #3.