Relative color syntax
The CSS syntax that lets one colour be defined in terms of another: take this one, take it apart into channels, change one of them, put it back together.
The construct is a from keyword inside any colour function, which binds the original colour's channels to names you can then do arithmetic on. Want the same colour at forty percent opacity and you write that channel as a new value and leave the rest untouched; want a darker variant for hover and you multiply the lightness channel. What makes it genuinely usable is the combination with modern colour spaces — in OKLCH a change in lightness actually corresponds to an evenly perceived change, unlike the same manoeuvre in HSL, which gives different results depending on hue. The consequence is that an entire theme can be derived from a single custom property right in the browser, removing one of the last reasons to run your CSS through a preprocessor.