The IntersectionObserverEntry interface's read-only rootBounds property is a DOMRectReadOnly corresponding to the target's root intersection rectangle, offset by the IntersectionObserver.rootMargin if one is specified.

A DOMRectReadOnly which describes the root intersection rectangle. For roots which are the Document's viewport, this rectangle is the bounds rectangle of the entire document. Otherwise, it's the bounds of the root element.

This rectangle is offset by the values in IntersectionObserver.rootMargin.