Along with the obvious solution of changing the source code directly, the package has a built-in mechanism for extending it by adding clauses to the following predicate:
The package has these two equivalences predefined (just to make environments more LaTeXish):
html_expansion(bf(X),b(X)). html_expansion(it(X),i(X)).
As an example, to define structures for the new HTML3 tabbing elements, one would define the following clauses:
html_expansion(settab(I),tabstop$[id(I)]). html_expansion(tabto(I),tab$[to(I)]).