When you visit an HTTP site, browsers give you a warning screen alongside an option to "open anyways".
We could do the same with sites that are not 100% correct. User are already used to having to click "Open anyways" for older, non HTTPS, sites anyways
Browsers briefly tried that in the early 00s. It turns out that, from a user perspective, that's an incredibly stupid question- the user has no way of knowing how well the page works until they click "yes"!