Animating custom properties appear to always end up non-composited (at least outside of CSS painting), and get lumped with all the other non-composited CSS properties found in the trace (don't be surprised like me and note that this happens regardless of what the property is animating, e.g. animating translate through fixed values is usually composited, animating --yheight in translate: 0 var(--yheight) will not be).

For example, you could end up with Unsupported CSS Property: --swing-y in non-composited-animations alongside other failed animations giving Unsupported CSS Property: color (or even in a list from the same animation, Unsupported CSS Properties: --swing-y, color.

It would be nice to separate the custom properties and give a little more context. You can probably figure out what it's referring to pretty easily, but it is a little weird to have these two types of things put together.

One solution we've discussed was splitting out any unsupported properties (probably here) beginning with -- and then giving a new failure string for them, e.g. Custom CSS properties cannot be animated on the compositor: --swing-y

These custom properties are Chromium-based browsers only right now, so low priority for this fix.