[web-animations-1] Propose moving iterationComposite to web-animations-2

[stephenmcgruer]

This is a proposal to move the iteration composite operation concept to the level 2 version of the spec. If adopted, section 5.5 would be removed, as well as all references to the iteration composite operation.

Motivation

This proposal is primarily motivated by the difficulty of implementing iterationComposite in Chromium (as our interpolation stack would have to change significantly to support it), backed by the claim that we would not lose too much utility in moving it to level 2. The goal is to clear the path for all web browsers to ship web-animations-1 in an interoperable way.

It is hard to judge what iterationComposite brings to the spec. It is not covered in the listed use-cases for web-animations-1. I was unable to find any JavaScript animation libraries offering a similar property in a quick search, but they may exist. If there are use-cases, I would be interested to hear them.

Based on the WPT tests, only Firefox implements iteration composite operations today.

Implications

Although less ergonomic, I believe web developers should be able to mimic the behavior of iterationComposite by setting the correct keyframes on the animation. (Please correct me if this is not the case.) So the main implication is a loss of ergnomics, if the web developer wants to do something iterationComposite-like.

[birtles]

The motivation for this feature is supporting accumulate="sum" from SVG. I don't think you can do that using other means but I also don't know that it is a commonly used feature so I think I would be ok with deferring to level 2.

[graouts]

I have no objection.

[stephenmcgruer]

Spec has been updated; remaining work is to figure out how to clearly differentiate the WPT tests as level-2. In #4303 @foolip suggested just using the existing meta tag, and then wpt.fyi might add an ability to filter by spec, however I was just thinking this doesn't really work for things like interface tests...

Still, I will take a look today to see what can be done :)

[foolip]

See web-platform-tests/wpt.fyi#65 for where that discussion goes, of if you want to make it go a particular way :)