That explains the incredibly attention-stealing animated knee joint ad I sometimes see on Apple News+.

On the other hand, the ads are usually static, the content on the page will stay put (unlike news sites on the regular web, where the paragraph I am reading will shift up or down and often will get completely jettisoned out of the viewport), there are no pop-ups, and the page has never scrolled back up to the top while I was already half-way down the article.