[csswg-drafts] [scroll-animations] ViewTimeline IDL specifies that the `subject` attribute is non-null, but allows construction with no `subject` specified (#13959)

weinig has just created a new issue for https://github.com/w3c/csswg-drafts:

== [scroll-animations] ViewTimeline IDL specifies that the `subject` attribute is non-null, but allows construction with no `subject` specified ==
The [ViewTimeline IDL](https://drafts.csswg.org/scroll-animations/#viewtimeline) specifies that the `subject` attribute is non-null

```
readonly attribute Element subject;
```

but allows construction with no `subject` set since [ViewTimelineOptions](https://drafts.csswg.org/scroll-animations/#dictdef-viewtimelineoptions) does not make `subject` a required member.

Later in the prose, it states:

"The values of [subject](https://drafts.csswg.org/scroll-animations/#dom-viewtimeline-subject), [source](https://drafts.csswg.org/scroll-animations/#dom-scrolltimeline-source), and [currentTime](https://drafts.csswg.org/web-animations-1/#dom-animationtimeline-currenttime) are all computed when any of them is requested or updated."

but I am not entirely clear on what it would be mean to compute the subject.

I think the spec probably just needs to update the IDL to make `subject` nullable.

(A similar conflict exists for the `startOffset`/`endOffset` attributes, and was filed separately as https://github.com/w3c/csswg-drafts/issues/13844). 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/13959 using your GitHub account


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Received on Saturday, 23 May 2026 22:18:54 UTC