Yet another clever technique from Lea. But I’m also bookmarking this one because of something she points out about custom properties:
The browser doesn’t know if your property value is valid until the variable is resolved, and by then it has already processed the cascade and has thrown away any potential fallbacks.
That explains an issue I was seeing recently! I couldn’t understand why an older browser wasn’t getting the fallback I had declared earlier in the CSS. Turns out that custom properties mess with that expectation.