I don't think that's been established. With the current state of advertising on the internet, it's not even possible to do this.

In general, websites use advertising networks which do not allow them to proactively vet the content. Even if they did, no amount of vetting can guarantee the content is benign (active content can do naughty things only some of the time or on some platforms, or things not yet recognized as naughty - this is also why antivirus isn't reliable). So, clearly the solution is to not allow Javascript or flash, right? Nope - exploits in image parsers, font parsers, video parsers, audio parsers, etc. come out fairly often.

This could maybe be dealt with by contracts between websites and advertising networks specifying that the advertising network will be liable for malicious content, but I don't see that happening.