In reply to mrphilipoldham:
Yes, very confident of where the notification came from because I've used the website notification functionality before and it shows which website generated the notification. I've also seen nothing like it before, and nothing more since I blocked the website. iOS is generally more secure than Android so it's quite possible that they don't try it on there, or don't succeed. Also possible that it's luck of the draw whether they actually try that tactic on you or not. Would make them much more likely to get away with it for longer. Or that it depends which adverts load on the page. As I said, I'm not blaming you, just warning other people.
Was it the first Google result or the top advert? Google are making it increasingly difficult to tell the difference and adverts are often more questionable results for some search terms.
This old forum thread suggests that the site used to be known for hosting questionable adverts, but wasn't necessarily out and out bad itself, which might explain both my experience and other people's lack of similar issues:
https://forums.malwarebytes.com/topic/230372-s5postimgcc-false-positive/