MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/backdrop-filter

What specific section or headline is this issue about?

First description of the backdrop-filter

What information was incorrect, unhelpful, or incomplete?

"to the area behind an element"

This is statement in the first paragraph is not correct

According to the specification the filter is applied to the area behind the element up until the nearest ancestor Backdrop Root element.

So that in this example: