My top list of recent CSS improvements: 1) Nested selectors. 2) :has(...). 3) :is(...), before you had to write :not(:not(...)). 4) :where(...), similar to :is(...), but the selector weight inside :where becomes 0. Useful when you need deep/complex selectors without increasing the selector weight.
I like 2-4 but I despise nested selectors. They make selectors ungreppable.
Why/how do you grep selectors? Seems overly optimistic to be able to guess the particular rule pattern that is applying a style. Browser tools are much more reliable.