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
As seen in this codepen: https://codepen.io/Claire-Chambers/pen/ZYbNeJy
The definition of backdrop root is specified twice. Once:
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.
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