[scroll-animations] ViewTimeline IDL specifies that the `subject` attribute is non-null, but allows construction with no `subject` specified

[weinig]

The ViewTimeline IDL specifies that the subject attribute is non-null

readonly attribute Element subject;

but allows construction with no subject set since ViewTimelineOptions does not make subject a required member.

Later in the prose, it states:

"The values of subject, source, and 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 needs to update the IDL to make subject nullable (and ensure all uses of subject by the processing model account for it potentially being null).

(A similar conflict exists for the startOffset/endOffset attributes, and was filed separately as #13844).

[BorisChiou]

A similar topic related to this: #9584

I think the spec probably needs to update the IDL to make subject nullable (and ensure all uses of subject by the processing model account for it potentially being null).

It seems WebKit and Chromium define element? subject in their idl files now. (And I'm planning to make the subject nullable in Gecko as well.)