AFAIK, all libraries are loosely coupled in Biff. Swapping out Hiccup / Rum for one of the other HTML templating options should be in "userspace"; straightforwardly so, without the framework maintainers intervention.

Ref:

Biff's own documentation: https://biffweb.com/docs/reference/architecture/#modifying-t...

HTML templating options: https://ericnormand.me/mini-guide/clojure-html-templates