For a discontinued external tool doing this, see WikiTrust. Showing that information would (IMO) be very valuable, but also a rather complex software engineering project.
Topic on Talk:User Interaction Consultation/Who wrote that?
Yes. This was talked about around 2 years ago but given wikitext compiles to HTML and doesn't give you a sound way to go the other direction this is technically hard to solve.
With parsoid this should be more possible but is still a large investment of work.
That said I have always thought this would be a useful and interesting feature.
I don't think this involves (or is helped by) HTML -> wikitext translation. It can be either be done by figuring out a way to "blame" pieces of wikitext plus figuring out a way to turn wikitext positions into HTML positions, or by blaming HTML history directly. In any case, WikiTrust did this (and more - they also counted editor and edit reliability ratings based on how often that editor's edits survived subsequent changes) well before Parsoid existed.
The task for this is T120739/T2639.
Possibly related: more intelligent diffs (T121469), content persistence research (IIRC @Halfak (WMF) is working on that currently to get editor productivity estimations).