HTML
Hypertext Markup Language (HTML) is what the web is built on. All pages make use of HTML when rendering out content. The language itself is not a true programming language, but it does provide a framework upon which CSS an JS can be embedded and run. CSS Index is what gives the web style while javascript index is what gives the web functionality and interactivity. There are many frameworks that can add additional functionality, but essentially, at the end of the day, most of that "fancy" stuff is transposed through a transpiler like Babel into standard Javascript that a web browser can understand.