<i>The problem is incorporating third party content that is neither screened nor sanitized.</i><p>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 hosting services, etc. work.<p><i>I think very few websites allow one person to embed arbitrary scripts that will be shown to another person.</i><p>Every single site on the web that hosts jQuery via a CDN does <i>exactly</i> that. This single example alone represents many millions of sites.