We don't have a proper way to search the OSM mailing lists so we should not ask people to do this until we do.
I feel telling people to not use HTML or not top post is a relic of a bygone era. Are folks really still bothered enough by this to make up rules for it in a CoC? I don't think it's necessary.

I am on the fence about the keep it short section for the same reason, but leaving it in for now.