[css-images] Clarify relationship between level 3 and 4

[dontcallmedom]

CSS Images 4 seems to be described as a superset of CSS Images 3:

It includes and extends the functionality of CSS level 2 [CSS2] and in the previous level of this specification [css3-images].

Yet, it doesn't seem to be actually a superset, e.g. object-position is not included in level 4.

A specific illustration of this is that its list of terms defined by references includes 16 terms from Level 3: circle, concrete object size, contain constraint, cover constraint, default object size, default sizing algorithm, ellipse, linear-gradient(), natural aspect ratio, natural dimension, natural height, natural size, natural width, object size negotiation, object-position, radial-gradient()

[LeaVerou]

Images 4 is a delta spec, it hasn't yet pulled in all of Images 3, it mainly includes the additions/differences.

[dontcallmedom]

thanks - it would be useful if that could be made explicit in the spec.

For instance, earlier versions of CSS Grid had the following language

This level is currently maintained as a diff spec over the level 1 module

[svgeesus]

Yes, it is certainly worth it for Images 4 to say

1. that is is a delta
2. that not all of Images 3 has been folded in

I recall, earlier in the development of Fonts 4, that Mozilla engineers were removing an already-implemented feature "because it isn't in the latest spec". So avoiding that sort of miscommunication can be vital.

[tidoust]

If level 4 is a delta spec, shouldn't https://www.w3.org/TR/css-images/ target level 3? It currently returns level 4 (cc @deniak)

This would also make things consistent with the drafts CSS server, as https://drafts.csswg.org/css-images/ currently returns level 3.

[deniak]

If level 4 is a delta spec, shouldn't https://www.w3.org/TR/css-images/ target level 3? It currently returns level 4 (cc @deniak)

@svgeesus @LeaVerou can you confirm the level 3 should be the "default" version?

[LeaVerou]

The thing is, it's mature enough and Images 3 is in CR, so it really shouldn't be a delta spec IMO.
@tabatkins @fantasai Perhaps it's time to de-deltify it? What do you think?

[fantasai]

Yeah, it's time to de-deltify.

@deniak I think Level 3 would be an acceptable default version for now, yes. I'll issue a request to update to L4 once it's stable and complete enough.

[deniak]

Thanks for the confirmation. https://www.w3.org/TR/css-images/ now points to the level 3.