With my testing for iOS 11 I’ve noticed that the untrusted ssl certificates now require more inputs from the user and safari behaves a lot like Google chrome.
Here are some examples:
iOS 10
User attempts to access a page with an untrusted ssl certificate
User just has to hit continue
iOS 11
User attempts to access a page with an untrusted ssl certificate
User has to now go to show details, and click on “Visit this website”
You can test the scenarios yourself by using https://badssl.com this site contains a number of examples of bad certificates that you can run through on the device.