The spec https://drafts.csswg.org/css-page/#page-based-counters says:

… The value of pages cannot be manipulated: while counter-reset and counter-increment statements that set it are valid, they have no effect.

However there are situations where resetting the pages counter may be useful, for example, when using content: 'Page: ' counter(page) ' of ' counter(pages); style page footer, and the first page is a cover page that should not be counted, you may want to reset both page and pages counters with counter-reset: page -1 pages -1 at the first page.

See the issue raised in Vivliostyle:

• Should we allow resetting the pages counter? vivliostyle/vivliostyle.js#1560