The definition of column-span says:

Applies to: in-flow block-level elements

The definition of block-level element says that it applies to things with display: block, display: list-item and display:table.

Then the definition of column-span goes on to say:

The element establishes a new block formatting context.

This means that (1) column-span: all applies to tables and (2) when it does, they become block formatting contexts.

Having a single edge case in which tables can establish block formatting contexts doesn't make sense. This shouldn't happen.

That said, given the lack of consideration to this issue, I wonder whether column-span is really intended to apply to tables.

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