Math font macros in superscripts now enclosed in braces.  Example:
1^\mathsf{W} becomes 1^{\mathsf{W}}.

This makes no difference to the resulting typeset material, but makes
it possible to use ^ as a *macro* rather than the math superscript
token.  There are some packages that make ^ an active character to
make superscript (and subscript) handling more flexible.  The ePub
conversion class is one of these.