The containing block definition says

a containing block is not a box (it is a rectangle), however it is often derived from the dimensions of a box. If properties of a containing block are referenced, they reference the values on the box that generated the containing block. (For the initial containing block, the values are taken from the root element.)

But what if something references a property of a containing block that has not been generated by any box, and is not the initial containing block?

According to CSS 2 and CSS Position, I think this case is not possible, but then you should say so. Otherwise, if it's possible, define what should happen.