Explain why <parallax-container> is needed

[domenic]

This may be more obvious when we have an implementation. But, it's not clear immediately why it's necessary, as opposed to making <parallax> elements work within any scrolling container.

If it's a matter of allowing the code to find the nearest scrolling container, that's interesting, as we've also seen similar problems in the virtual scroller design, so that might point to a missing primitive.

[Malvoz]

FWIW, I believe with Scroll-linked Animations there's no need for a <parallax> element.