http://www.certera.io redirects properly to https://certera.io.

https://www.certera.io fails the certificate check.

It's a good example of the difficulty of getting TLS perfectly right.

In theory this set up is fine; the default behavior of all the browsers when typing "www.certera.io" is to interpret it as a request for http://www.certera.io.

But if the client has anything in place that automatically upgrades http to https before submitting the request, you're going to need a valid cert for the www subdomain in place or you'll throw a cert error before reaching the redirect.

Even if your site omits the www subdomain in production (as certera does), a lot of users will just type it in anyway. So, you better be ready to handle that request via https.

You're spot on. I was aware of this limitation of GH pages and once I make money, I can start spending on actual hosting. I explained more above.