Malicious site appears in Safari

A completely unsolicited web site, “happy.uslucky.club”, appeared in the Safari browser. This is not a popup, but afull web site with no option to close the page without clicking “ok”. If I close the page normally, it just comes back. The URL contains a large amount of code, possibly an overflow attack? I got rid of the site by entering the URL as an Never Allow adult site in settings.

How did this mess get through and execute on Safari? How do I find out what else it did on an O/S that doesn’t allow malware scanners?