[filter-effects-2] Ambiguity in spec over whether root elements of child iframes form Backdrop Root #610
Description
As seen in this codepen: https://codepen.io/Claire-Chambers/pen/ZYbNeJy
The definition of backdrop root is specified twice. Once:
A Backdrop Root is formed, anywhere in the document, by an element in any of the following scenarios. See [§ 3.2 Backdrop Root Triggers](https://drafts.fxtf.org/filter-effects-2/#BackdropRootTriggers) for more details on each:
* The root element of the document (HTML).
...
Which would seem to imply that the root element of an iframe should be a backdrop root, since every iframe has its own document. However, section 3.2 backdrop root triggers merely states.
Obvious triggers:
The root element. There is nothing above the root element, so it must form a Backdrop Root.
which could imply that only the root element of the root document should be a backdrop root, as document roots of iframes still have something 'above' them and therefore don't have to form a backdrop root.
As it stands, chrome and firefox both treat iframes like any other element, ie, not a backdrop root. There's no technical reason iframes should form a backdrop root, since iframes are already allowed to have transparent backgrounds and will quite happily allow its parent frame to shine through, if the developer wants it.
That being said, I certainly thought it was 'weird' when I discovered this, and it's definitely somewhat non-intuitive. Regardless of which behavior is better, the spec should be clarified.
cc: @progers