Stack
HTML
5.0 / 5
The markup language that structures the entire web. Everything else (styles, interactivity, frameworks) rests on it; writing it well still makes the difference.
HTML is the layer that never moves: it defines what each thing on the page is (a heading, a list, an article), and accessibility, SEO and now also whether AIs understand your content all depend on that semantics. This very site is designed from semantic HTML upward.
You learn it in an afternoon and master it over years: the difference between div-soup and a well-structured document isn't visible at a glance, but screen readers, search engines and anyone who inherits your code notice it.