Earlier quoted context omitted.
I agree and disagree. From the perspective of anyone making a web site it's very easy to secure yourself against JS running on a third party domain: don't load any. That sites do load these scripts says a lot more about their priorities and the state of online advertising than it does about browsers themselves.
But the problem is that the server is deciding for the user. If they want to show me sketchy ad content, they can go fetch it and send it to me as part of my request to them. Don't tell me to go get it myself. If an ad server is malicious, let it be the web server that has to deal with them, not me.
That's what I call server deciding for the user. And now you're in real trouble with security.