I assume you mean "reset", as opposed to "cancel".

Now that I think about it, it would be better to create new standard inputs that can be used on all forms, instead of just co-opting "cancel" only for Special:RunQuery forms. In that case, I would expect a "reset" standard input to reset the form to its original, preloaded condition. And, a "clear" standard input would blank all fields.

So, I suppose that's 2 different new standard inputs needed to cover both resetting the form, and for clearing it.

Setting this to "wontfix" - I think "reset/clear" buttons in forms are generally bad; I agree with this:

http://www.nngroup.com/articles/reset-and-cancel-buttons/