Dave Ward says,
It’s not exactly light reading, but section 4.2 of RFC 3986 provides for fully qualified URLs that omit protocol (the HTTP or HTTPS) a
If you would like to make sure all requests are upgraded to secure protocol then there is simple option to use Content Security Policy header upgrade-insecure-requests
Content-Security-Policy: upgrade-insecure-requests;
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/upgrade-insecure-requests