To be clear, the skills document how to use these features to solve common use cases, and then they give your agent a full browser support table for any APIs mentioned that are not Baseline Widely available -- and then your agent can decide whether to use them or not, based on your site's browser support requirements.
In most of these cases a cross-browser fallback is also documented.
"DO use fetchLater()..."
"Your own code MUST call fetchLater() directly..."
https://github.com/GoogleChrome/modern-web-guidance/blob/mai...
But yes, the skills do document compatibility and fallbacks well.
The mental model we've been using on the team is:
If you hired an expert web development consultant to build X, what would they recommend as a best practice? When does it make sense to use the new API (because it provides real benefits) vs. when does it make sense to wait until it's available everywhere?
To use your `fetchLater()` example, this is a case where the `fetchLater()` API legitimately solves a very real problem for people trying to build accurate analytics or telemetry features, see: https://nicj.net/beaconing-in-practice/
And lots of companies actually are already using `fetchLater()` in production now because of its increased reliability.