[css-gcpm][css-content] "string-set" on elements with no boxes: How to determine which page the assignment is on

The string-set property applies to "all elements, but not pseudo-elements", which includes elements with no boxes (e.g. via display: none).
Regarding such elements both specifications say ( after https://drafts.csswg.org/css-content-3/#valdef-string-first-except ):
"The content values of named strings are assigned at the point when the content box of the element is first created (or would have been created if the element’s display value is none)."

Determining where a box "would have been" is rather complex, as that could be influenced by it's height (near a page break) which in turn could be influenced by it's content.
Adding that the hypothetical box should be considered to have a block-size of 0 would solve this for simple cases.

However in more complex contexts this would still not be easy to determine, e.g. when the order property is involved.
We would propose to go backwards through the DOM from the element until an element is found that has boxes in the content flow.
The page that the last of those boxes is on is the page of the assignment.
If coordinates on the page are also required they should be at the block-end of that box.
However, we would prefer that inside the page the order of assignments is strictly the DOM order.

Start with the CSS Content specification section linked in the issue, especially the rule for assigning named strings when an element has no content box. Examine the display:none, page-break, and order cases described there. Done means reaching and documenting an unambiguous rule for the assignment page and ordering of assignments.