This is in the 17 September 2013 Editor's Draft.

In section 1.3, we have a nice example of roll-up regions:

Region: id=fred width=50% lines=3 regionanchor=0%,100% viewportanchor=10%,90% scroll=up
Region: id=bill width=50% lines=3 regionanchor=100%,100% viewportanchor=90%,90% scroll=up

However, since each has width 50%, but are anchored 10% away from the edge, the regions will overlap. The example should use width=40%