RooMultiCatIter is unnecessary, since all multi category classes know how to generate their own states. Use begin() / end() and range-based for loops on instances of these classes.
This function is unneccessary if the template parameter is RooAbsRealLValue (+ derived types) or RooAbsCategoryLValue (+derived types), as arg() will always return the correct type.
We keep THtml for those who still need it for legacy use cases. ROOT has since several years moved to doxygen as documentation generator. THtml is not developed nor supported anymore; please migrate to doxygen instead.