I don’t know either if lack of a slash after domain makes for an invalid URL, I think they will just work, similarly you can just type “time.gov” into the address bar and the browser knows to try HTTPS on port 443 and HTTP on port 80, and request the document path “/” (explicitly, this is “https://time.gov/”). Lemmy and Fennec automatically add trailing slashes to them, apparently. However, you can cheat that by creating a hyperlink whose display text is “https://time/.<zwsp>gov” where “<zwsp>” is a zero-width space.

By the way, “www.” is a subdomain like any other, but people tend to add/remove it at will so it is considered good practice to make a redirect, or point the DNS A (IPv4) and AAAA (IPv6) records to the same server, and mark one copy as “canonical” (this is required by search engines). Yet, there are many servers that only work with or without “www.”, and possibly some where the content differs.

Edit: that “explicit” URL got the port (:443) edited out by Lemmy!