Normally replacing the attribute is best. But I do want the ability to merge CSS
classes, so an option that merges words lists would be useful (ie turn "one two"
and "two three" into "one two three").

I don't think it is useful to support merging of style attributes. Anyone using
those instead of proper CSS deserves the pain.

I'm wondering if it would be useful to support removal of attributes. I can see
a use for doing things like removing js events (ie remove onchange from an input
element) but I don't think
that is a high priority. This may already be possible with the existing drop rule.