AFAIK it used same NeXTStep Text (NSText) API [0] that is still being used on macOS nowadays [1], it didn't support rendering images at first:

> The inline images such as the world/book icon and the CERN icon, would have been displayed in separate windows, as it didn't at first do inline images.

Historically, DPS is one of the reasons Safari PDF exports look so good: Apple based CoreGraphics APIs on DPS to make the migration from NeXTStep to Mac OS easy. This makes the CoreGraphics<->PS/PDF conversion fairly straightforward.

[0]: https://www.w3.org/People/Berners-Lee/WorldWideWeb.html

[1]: https://developer.apple.com/documentation/appkit/nstext