There are two options:

* Manually install a root certificate, which is a confusing process for most end users and a non-starter for anyone who cares about security. (Imagine walking your parents through the process.)

* Trust a self-signed certificate, which is an increasingly difficult and counterintuitive process since Chrome and Firefox started competing to see who could destroy their usefulness faster. I'm not even sure if it's possible anymore.

Neither of these are acceptable.

I mean I’m not sure if there’s a solution that will make everyone happy then. Making trusting self-signed cheers easy and not scary has real security implications because users just click-through warnings.

Making casual users create their own root certificates sounds like an even worse problem. Now an attacker isn't restricted to impersonating your lightbulbs. They can impersonate any domain if they can get your private CA. Now imagine if an IoT vendor engages in questionable practices like creating the CA for you and the user only has to download an exe that automatically installs the root certificate. The benefit for the vendor would be that all devices you order from their website would be shipped with correctly signed https certificates. Later a hacker dumps the database with root CAs and uses it to impersonate your bank.

That’s why self-signed and "local signed" should be distinct concepts, IMHO. The .local domain is already special cased, and could provide a different UI path more akin to how SSH works. AFAICT, you can’t get a https cert for a .local domain, so it’d not break existing https security model. It’d provide a more secure way for apps like syncthing to provide a secure local UI as well. Getting browsers to accept my self-signed certificate is a pain and makes people just use http.

Honestly, I don't trust most end users to install root certificates

If you are doing something for an end user, I think it makes a lot of sense just to get a certificate; it's just not a large barrier anymore.

The mechanism SSH uses is called Trust on First Use ("TOFU") and is closer to what used to be HTTPS certificate pinning. In this scheme, certificates never expire, and if they do, clients warn about the unexpected change in certificate.

It is different from the CA PKI system, where the client trusts any certificate signed by a trusted CA without prompting the user at all, and doesn't prompt the user if the certificate for a site changes.

Self-signed certificates give you basically this. It's a bit of a hassle to mark them as trusted, but you only have to do it once.

That has not been my experience with current versions of Chrome.