Earlier quoted context omitted.
The problem is incorporating third party content that is neither screened nor sanitized. The proportion doesn't matter as much as the fact that there is nothing stopping attacks. They only get cleaned up afterwards. You don't expose your users to attack without warning. I think very few websites allow one person to embed arbitrary scripts that will be shown to another person.
The problem is incorporating third party content that is neither screened nor sanitized. It is impossible to screen or sanitize third party content if the third party is hosting it and the user loads it when your page refers to it. The third party can change that content at any time, without your knowledge or consent. This is how almost all ad networks work. It is also how almost all CDNs, web font services, image ho…
I'll repeat myself. "I think very few websites allow one person to embed arbitrary scripts that will be shown to another person." This is not happening as a result of you using an image host. No scripts are involved there. This is not happening as a result of the site using a CDN. No user triggered that load of jQuery.
It's fine to load jQuery from a specific server that you trust. It's also fine to load ads from the ad network's server, as long as they are policing uploads properly. The problem is they usually don't.