I continue to believe that <progress> is really just a specialized version of <meter> as it's currently defined, and that simple CSS properties could be used to style <meter> into a native progress bar using a slight extension CSS3-UI. However, there is a way to differentiate the two. Progress bars can have an indeterminate state. Why not give them an |indeterminate| attribute?
(Are active and inactive states necessary for an element that only serves as an output?)