named-pages select first page issue

[SunALan0]

Describe the bug
When using named-pages, I attempted to change the footer content displayed on the first page of 'Page B' using the method described below, but it did not work as expected.

page-groups.html

page-groups.txt

[MurakamiShinyu]

Unfortunately, the page selector :nth(… of …) is not yet supported. See Vivliostyle's Supported CSS Features document:

https://docs.vivliostyle.org/#/supported-css-features#css-generated-content-for-paged-media-gcpm-3

• The :nth(An+B) syntax is supported but the :nth(An+B of <custom-ident>) is not yet supported.