HTML
In plain terms
The markup language that defines the structure and content of web pages.
Going deeper
HTML uses tags to label content as headings, paragraphs, links, images, and forms, giving a page its skeleton. Browsers read it to build the page, which CSS then styles and JavaScript makes interactive.
Under the hood
Semantic elements (header, nav, article) improve accessibility and SEO; the browser parses HTML into the DOM tree, and malformed markup is error-corrected rather than rejected.
Examples
<div>, <form>, <video>, <table>; semantic tags <section>, <aside>, ARIA roles.
Think of it like...
Like the framing and rooms of a house: it defines what goes where, before paint (CSS) and appliances (JavaScript) are added.