Yes, I see.... Also sorry for the last comment, I didn't notice that input attributes (size, maxlength etc.) can be passed with add_attributes. I think htmlfill is very usefull right now, but just out of curiosity: do you think it needs some extra functionality? Do you miss something in htmlfill already? :)
Missing... well, there's the automatic error insertion, which I noted before. It would also be nice if there was a purely information parsing option. Like, you'd be able to put an attribute in like form:required="true" and then parse that out of the form and create a validation schema from that (using whatever validation system you chose). When actually rendering the form, all those attributes would be removed. I think that's mostly it.