> Do we even need *any* element for this? What's wrong with U+00B2? Think
> of the poor screen-reader programmers.

It's essentially presentational when used for many purpose.  When 
digits are used as an exponent (or even as a crossreference) they 
represent parts of a number in a special context, not a string of
a completely different type of character.

E.g. in "octane has the chemical formula C<sub>8</sub>H<sub>18</sub>,
18 is the number eighteen, not the character string superscript 1 followed
by superscript 8.