[web-animations-2] Propose a new timing option `align` to replace `SequenceEffect`

[ydaniv]

Currently the synchronization of child animations in a GroupEffect is specified by either using a GroupEffect or a SequenceEffect as demonstrated in the spec.

I propose simplifying this approach with a new property in the EffectTiming options with proposed name align (TBB) that controls alignment of child animations in the group, with the possible following values:

• start: all children start at the effective start-time of the group. This is the default value
• end: all children end at the effective end-time of the group, while also respecting endDelay.
• sequence: each child starts sequentially after its preceding sibling effect, just as currently defined for SequenceEffect.
• sequence-reverse: same as sequence but the order of siblings is reversed.

IMHO this method should also make the definition of GroupEffect more clear and separated from the concern of child synchronization.
The idea here is also to align the JS API with the proposed CSS API by using the same longhand property name. Otherwise, using a different class may translate into a separate shorthand like sequence-effect, or otherwise become awkward.

[birtles]

This is a good idea. It's more consistent with CSS behavior and I think that anything makes our animation properties/APIs feel more like temporal layout ("flexbox for time") is probably a good step towards creating a more coherent platform.

For what it's worth, I think we arrived at the current API design due to following the precedent of other animation APIs like SMIL (which has <par> and <seq> elements) and QML (see wiki). I agree we should make the JS API match what we do in CSS and drop the separate SequenceEffect class.