That's the value given in the spec, using None doesn't add anything and even
introduces bugs.

With the previous system, we were listing all page types as the cartesian
product of page names and pseudo selectors. Declarations were added for all
these possible page types.

The nth() page selector makes this system impossible, as we can't make the
cartesian product with a number of pages we don't know at the beginning of the
rendering. We now add declarations before rendering each page, based on the
properties of the page.

This new way is faster when the total number of pages is lower than the
cartesian product of possible pages. For long documents, we could improve speed
by detecting CSS rules based on page index or page name, and use cached results
for all pages that don't rely on such rules.