Yea this would be a welcome addition.

Progress on this? Best I could come up with in the meantime:

html="<h1>hi</h1><h2>there</h2>"
diff --unchanged-group-format="" \
  <(echo "$html" - | pup --indent 0 'h2') \
  <(echo "$html" - | pup --indent 0 '*')` | pup

--indent 0 is redundant in this example, but in multiline html changing indents will create no match, so I guess you could say... it makes a diff (sorry). The final pup pipe will re-indent the html for ya

@nick-bull #81 implements this. I see that Eric has just started making commits again, hopefully he'll merge the open PRs soon and make a new release. In the meantime pulling and building my branch should get you what you need.

I merged this into https://github.com/frioux/pup, fyi