CSS doesn't have a notion of page spreads, but the float-defer properties introduced in Prince 14 makes it possible – even easy – to place content over a spread. The trick is to (a) duplicate the content in the source, and (b) carefully place the first element in the inside column of the left page, and (c) place the second element in the inside column of the right page. To see only the right part on the right page, the second element is (d) given a negative margin.

It's actually easier to express this in CSS than to describe it in English. Here are samples:
https://www.princexml.com/howcome/2021/guides/float/#spreads