[Page DOM] page counters across multiple documents

Most book-like documents consist of multiple HTML files. Consider a tiny
book with two HTML files, one for the first chapter and one for the second
chapter. We'd like to display page numbers throughout the document, so
chapter two would need to know how many pages were in chapter one.

CSS counters can be initialized, but we'd need some method to obtain
information about previous and subsequent files in the same book, and pass
that to the page and pages counters. This could get expensive...

Dave