Found it: https://forums.opera.com/topic/25957/am-i-infected-by-a-malware.
It's very weird and I feel not safe having this scam websites added as CSS injection in my pages source code. I see porn website URLs everytime I do some debugging, or I have to display ads... of course I can deactivate the ad blocker on sites I am developing on, but still a very strange dilemma.
Thanks for your help @leocg