[whatwg] <progress> element and indeterminate states.

[Matthew Raymond]

   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?)