/* “Invert” other pseudos/counts: all non-first `p`: */
p:not(:first-child) { color: red }

/* Or check elements: any child that isn’t `p`: */
section *:not(p) { color: blue }

/* “Flip” a `.class`: `section` that aren’t `.warning`: */
section:not(.warning) { background-color: lightgray; }

/* But often best to just target things themselves! */
section:first-child { background-color: lightgray }
.warning { background-color: yellow}