[animation-triggers-1][editorial] / must be written literally

[cdoublev]

6. Slashes (/) [...] are written literally. Other delimiters must be written enclosed in single quotes (such as '+').

https://drafts.csswg.org/css-values-4/#component-types

Note that CSS V&U could also say:

Delimiters must be written enclosed in single quotes, but slashes and [...] can be written literally.

If not explicitly allowed, it seems legitimate to consider a slash written between quotes as invalid, although it was valid before.

As always with the CSS value definition syntax, the UX via @property/syntax and custom function parameters should probably be considered.