Malware Problem—Safari Page Won't Close

I followed a link from a youtube video and a suspicious site downloaded itself. It immediately asked me to allow notifications and 'Not Allow' would not function. The page will not close. When I force quit Safari and reopen, the page reappears. I have tried emptying the cache and deleting the two suspect cookies but this does not help - the cookies reappear. Shutting down and restarting is ineffective. Malwarebytes doesn't detect anything.

Images show part of Safari page and web address, which has since changed to 6 rather than 5.

I'd be grateful for any suggestions.