[css-multicol-1] balancing and overflow columns #2549

The spec says

In continuous media, this property does not have any effect in overflow columns.

However, Gecko and Edge interpret that as “This property does not have any effect when there are overflow columns” and Blink ignores the sentence entirely. Probably we should spec a behavior that's implemented (and less self-inconsistent than “balance non-overflow columns, but don't balance overflow columns”).