Should I down-vote an accurate answer that lacks references?

Question

I commented on this answer to a question about the long-ago evolution of the thorn (þ) into a "y".

@Jon Hanna answered the question very well, with an answer that is, to my knowledge, perfectly correct. The issue arises in the fact that he did not quote any sources nor cite references.

On links, the EL&U Help Centre explains the following:

Provide context for links

Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.

I am not sure whether I should down-vote the answer for not providing references, up-vote it for being correct or do nothing and search online for corresponding links so that I can add them for myself. I should note that the Help Centre uses the word "encouraged" in relation to this topic, do perhaps down-voting simply for a lack of references is wrong.

How should I respond?

Answer 1

Links to external resources are encouraged, but not required.

Upvote useful answers. Downvote answers that are not useful. A useful answer is an answer that will help not just the asker but also anyone who might come across the question in a site or Google search.

It's fine to improve someone's answer by adding links to useful external resources.