The definition of column-span doesn't appear to say anything about what happens to the elements that are ancestors of the element with column-span: all but descendants of the "nearest multicol ancestor in the same block formatting context" (i.e., the element that establishes the columns being spanned).

What happens to the backgrounds, padding, borders, and margins of these elements where the column span interrupts them? Do they appear as though they were a block ancestor of the column-spanning element? (What if they were inline elements?) Does relative positioning on these elements apply to the column-spanning element? Transforms? etc.

This situation is in some ways analogous to the situation that happens when an inline contains a block, which is defined reasonably thoroughly in CSS Level 2, except that this situation is a bit more general since the ancestors being split can be either inlines or blocks, whereas in the block-in-inline situation the split ancestors are only inlines.

I'd note that failing to display the background of an ancestor behind the column-spanning element could violate accessibility expectations related to colors and backgrounds.

What do existing implementations do? (IIRC, Gecko is the last engine to implement column-span, which would have made me expect the spec to be in better shape than it appears to be.)

/cc @tabatkins @gregwhitworth @neerjapancholi