CSS3

The skill is more structural than it looks. Layout with grid and flexbox, a responsive approach that holds from phone to wide monitor, custom properties that keep a colour scheme in one place, and animation that stays smooth because it moves the right properties. On a long-lived interface, the discipline that keeps styles from accumulating matters as much as the styles themselves.

Accessibility and dark mode belong here too, and both are far cheaper to build in than to retrofit. So is testing in the browsers people actually use rather than the one on the developer's machine.

HTML5 covers the browser platform as a whole, and JavaScript frameworks handle behaviour. CSS decides what the result looks like and whether it holds together as it grows.