https://drafts.csswg.org/css-gcpm-3/#document-page-selectors has this to say on page groups:

When the page property is applied to an element that also has a forced break property applied, a page group is created.

But examples 13 and 14 immediately below that don't show a forced-break. Without this, neighbouring div elements with the same page attribute could be on the same page.

To fix example 13:

• Change div { page: A } to div { page: A; break-before: page }

To fix example 14:

• Change div.Chapter { page: body; } to div.chapter { page: body; break-before: page } (note case-change of "chapter")

Also, the specificity of the "nth" selector is undefined. I presume :nth(5) is (0,1,0) and :nth(5 of A) is (1,1,0)?