The “Baseline” initiative now has a useful suggestion: their “wider support” high compatibility status is “all major browsers have been shipping it for 2½ years”. See https://github.com/web-platform-dx/web-features/blob/main/do... for more details. (When they first announced Baseline, its definition was disastrously bad, generally only covering ⅔–¾ of actual users, but I’m pleased they’ve listened to feedback and redone it completely; the definition it has now is a little more conservative than I’d have gone for—I’d have gone 2 years—but that makes it even more dependable, where that status is achieved.)

Picking jut a couple of these specific features:

• CSS Nesting as a whole is only 4–8 months shipped, and the “relaxed nesting” only shipped earlier this month in Chromium and Safari. At this point, you should absolutely not “drop those extra &”, and almost certainly not depend on nesting at all yet. And since it’s syntax, it’s not really a progressive enhancement candidate. So this becomes just a “don’t ship nesting syntax for another year or two”. Author with it and transpile down to compatible syntax with something like Lightning CSS, by all means.

• color-mix() is only 7–12 months shipped, though as a saving grace it’s Safari that’s the oldest (Safari is the browser most commonly far out of date in features due to Apple’s tie-browser-updates-to-OS-updates approach). It’s not generally a good idea to depend on color-mix() yet, though there are some progressive enhancement cases where it’s reasonable. For example, `text-decoration-color: color-mix(in srgb, currentcolor, transparent 50%)` makes underlines semitransparent, and it probably doesn’t matter if the browser skips that.