Is it possible to slightly manipulate the output HTML?

[sindoc]

Is it possible to use pup to add HTML elements under a particular node?

Say I need to add under 'html > head'

[gromgit]

No. pup only selects and outputs existing HTML. It's also pretty much abandoned, so I wouldn't hope for enhancements at this point.

You probably need something like XMLStarlet instead. It's even older than pup, and it's really intended for XML manipulation, but it should work OK with HTML that's not completely sloppy.