I had maintained an additional blog besides this one, emphasizing aspects of security technology. I hadn't posted there in a while, and decided to consolidate its content here alongside other posts. Since it was a security-oriented blog after all, it had seemed like an obvious Right Thing to set up the options so that it was accessed (or at least accessible) via an https:// url.
I closed the account on the old blog's hosting provider, and wanted to redirect any visitors to a landing page that would inform them of the change and point them at the blog you're now reading. I went to my domain registrar's admin UI, and had no problem setting up such a redirect for the http:// form of the old blog's url. The plot thickened for the https:// form, though. Reasonably enough, you have to have a certificate corresponding to a domain (and, of course, the ability to wield its corresponding private key) in order to deliver valid, authenticated content for that domain via https. And, an https redirect is a (small, but significant) example of such content; if you're relying on the security https is designed to provide, you wouldn't want an attacker without the appropriate key to be able to mislead by redirecting you elsewhere.
I could probably have arranged to get a certificate enabling my domain registrar to issue a valid https redirect to my domain's landing page, but that seemed like a lot of work just to support redirection rather than more comprehensive hosting. It was easier to solve the problem by pointing the domain's name servers to where I'd put the landing page, at a provider that was equipped to serve that page whether accessed via http:// or https://. (Thanks again, FastMail.)