[css-variables] allowable variable names

The syntax of var() is defined as:

   <variable> = var( variable-name [, <any-value> ]? )

What is the syntax of a variable-name here?  If it is meant to be an 
〈ident〉 then that should be stated.  (I think it probably is, given 
the note that begins with the text "Authors are recommended ...", but 
best to be clear.)