How to create a PDF with dynamically-sized header that is different on subsequent pages from HTML using CSS, Thymeleaf, and FlyingSaucer?

Question

I'm developing a tool which allows generating PDFs from HTML templates using FlyingSaucer's OpenPDF library, which supports CSS 2.1.

The PDFs being generated need to have a header at the top of each page. That header includes user-provided content, so the height varies.

However, we want a reduced header that displays fewer fields on subsequent pages.

Initially, I was putting the header into a <thead> element and forcing it onto each page with -fs-table-paginate: paginate in the CSS.

However, to support showing a reduced header on subsequent pages, I switched to the following CSS:

    display: block;
    text-align: center;
    font-size: 50pt;
    position: running(first-page-header);
}

div.header {
    display: block;
    text-align: center;
    position: running(header);
}

div.footer {
    display: block;
    text-align: center;
    position: running(footer);
}

@page :first {
    @top-center {
        content: element(first-page-header)
    }
    @bottom-center {
        content: element(footer)
    }
}

That allows the header to include different content on subsequent pages, but the height of the header and footer are fixed to a certain number of pixels. I can adjust that by setting a margin attribute on the pages, but I either leave a bunch of empty space or I risk the header text overlapping with the body text.

I know it's possible to inject page numbers using CSS, which seems like it might possibly lend itself to rendering different content in the header depending on the page number, but I suspect that's not going to be a good solution.

Possibly there's some trick with :first-child that would allow rendering the first page's header differently?

If it helps, these are actually not just HTML, but Thymeleaf templates; that's how I'm injecting the user-provided content. However, since the conversion to a PDF with pages is after the Thymeleaf step, it doesn't seem likely to help.

I don't think rendering to PDF, measuring the height of the header, and then setting the top margin accordingly on a second render is a particularly good strategy.