Specify the mapping of containing block to element

With respect to the proposal to use "nearest containing block ancestor" as the way to aggregate text nodes:

The containing block is a rectangle, not an element, and the mapping between rectangles and elements is not trivial

1. a single element can map to multiple containing blocks,
2. a single containing block might not map to any specific element at all (e.g. this is what you get if a table-row cell contains several block-and-text children: you get an anonymous table-cell box which determines the containing block for those blocks-and-text, but that box does not correspond to any elements).

Can we specify the mapping from containing blocks to elements?

Start by reading the proposal's use of the nearest containing block ancestor and the CSS containing-block cases described in the issue. Compare element-to-box and anonymous-box scenarios, then define an unambiguous mapping that covers the listed cases. Done means the specification states how containing blocks map to elements.