The xeplist page says Prosody supports 156 because it uses DNS records, which is maybe technically true, but not helpful when wanting to set it up for webchat.
For webchat it's also necessary to create and serve up host-meta and host-meta.json files.
Would be good if that would be reflected somehow in this doc.
FYI, after looking at the XEP list, I found and followed this guide:
Thanks. I'm not sure if a full blown guide fits in the 'notes' column on this page, nor do I really know what to write there right now.