For most people, the attention-getting devices used in Ads or to provide stimulation on websites act at this tolerable level. They tickle your attention circuits, but don't kidnap your train of thought.
Some people have a lower threshold for distraction than others.
And this condition can be a disability if they can't manage the quiescence vs. distraction level in their environments. What is done in private between a consenting person and their computer should be more manageable than that. It would appear that the User Agent guidelines will call for User Agents to give users override control to still squirmy pages. So the content guideline in this area might be something like the guidelines about color. Do not rely on motion alone to draw attention to a featured element.