[fxtf-drafts] [filter-effects-2] Ambiguity in spec over whether root elements of child iframes form Backdrop Root (#610)

Maya-Fey has just created a new issue for https://github.com/w3c/fxtf-drafts:

== [filter-effects-2] Ambiguity in spec over whether root elements of child iframes form Backdrop Root ==
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 [ยง 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 

Please view or discuss this issue at https://github.com/w3c/fxtf-drafts/issues/610 using your GitHub account


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Received on Thursday, 11 September 2025 09:27:02 UTC