[scroll-animations] Ignore unknown descriptors

[andruud]

We should allow (but ignore if unknown) any descriptor, otherwise it's a problem for forwards compat.

The <declaration-list> inside of @scroll-timeline rule can only contain the descriptors defined in this section.

Could be changed to (borrowing from css-properties-values-api): "Unknown descriptors are invalid and ignored, but do not invalidate the @scroll-timeline rule".

[andruud]

cc @majido

[majido]

I think css syntax in general does not throw away invalid declarations so it is forward-compatible that way.

I looked at a few example of at-rules definition and cargo culted this wording e.g.,

• @font-face

  The @font-face rule accepts the descriptors defined in this specification.

• [@Viewport]

  This section presents the descriptors that are allowed inside an @Viewport rule. Other descriptors than those listed here will be dropped.

• [@font-face]

  The @font-face rule accepts the descriptors defined in this specification.

Your suggested wording from css-properties-values-api seems that way mostly because there is wording prior to that that certain descriptors are required and otherwise the rule itself is considered invalid.

@Property rules require a syntax and inherits descriptor; if either are missing, the entire rule is invalid and must be ignored. The initial-value descriptor is optional only if the syntax is the universal syntax definition, otherwise the descriptor is required; if it’s missing, the entire rule is invalid and must be ignored.
Unknown descriptors are invalid and ignored, but do not invalidate the @Property rule.

Having said that changing the wording to be more explicit is fine with me. e.g.,

Only the descriptors defined in this section are considered valid in the inside of @scroll-timeline rule. Unknown descriptors are invalid and ignored, but do not invalidate the @Property rule.

[andruud]

Agree that suggested wording might have been too css-properties-values-api-specific. As long as we agree that the rule isn't invalid with unknown descriptors, it's fine. (We can WPT it). :-)

The reason I reacted is there's a difference between:

The @font-face rule accepts the descriptors defined in this specification.

and

The <declaration-list> inside of @scroll-timeline rule CAN ONLY contain the descriptors defined in this section.

The latter begs the question of what happens if you violate that requirement.

[cdoublev]

We should allow (but ignore if unknown) any descriptor, otherwise it's a problem for forwards compat.

Do forwards-compat requires that ignored properties/descriptors should be exposed in CSSOM (eg. via CSSRule.cssText for descriptors)? Related: #2736.

Browsers seem to interpret these wordings differently:

<style>
  @page { /* "The @page rule can only contain page properties"... */
    top: 1px; /* ... `top` is not one of them */
  }
  @keyframes animation {
    from { /* "The <declaration-list> inside of <keyframe-block> accepts any CSS property except those defined in this specification" */
      animation-name: 'foo';
    }
  }
</style>
<script>
const { styleSheets: [{ cssRules: [{ style: pageStyle }, { cssRules: [{ style: keyframeStyle }] }] }] } = document

console.log(keyframeStyle.animationName) // Chrome: 'foo' | Firefox: ''
console.log(pageStyle.top) // Chrome: '1px' | Firefox: ''
</script>

[tabatkins]

The Syntax spec is clear on this:

For rules that use or , the spec for the rule must define which properties, descriptors, and/or at-rules are valid inside the rule; this may be as simple as saying "The @foo rule accepts the properties/descriptors defined in this specification/section.", and extension specs may simply say "The @foo rule additionally accepts the following properties/descriptors.". Any declarations or at-rules found inside the block that are not defined as valid must be removed from the rule’s value.
[...]
For rules that use , the spec for the rule must define what types of rules are valid inside the rule, same as , and unrecognized rules must similarly be removed from the rule’s value.

So, unless otherwise explicitly stated, unknown/invalid descriptors, properties, or at-rules encountered in any at-rule do not invalidate the whole rule.

[tabatkins]

Browsers seem to interpret these wordings differently:

Yeah Chrome is buggy here.

[flackr]

The scroll-animations-1 spec no defines any at-rules so I think this is obsolete?

[fantasai]

Invalidated by further edits...