Indeterministic progress bar

Hi,
Reading through the section "UA Requirements for showing the progress bar" in <http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#the-progress-element>, UAs are required to clearly distinguish between determinate and indeterminate progress bars.

Does the working group have some recommendations regarding what should the indeterminate progress bar look like?
I would imagine that e.g. both determinate and indeterminate progress bars should have the same size, to avoid re-layout when an indeterminate progress bar turns into a determinate progress bar.
thanks, Yael