This MDN page explains the basics of what HTML is, and what HTML elements are. It also provides links to tutorials and other references in order to learn more.
This page explains how CSS is used alongside HTML to style web pages. It provides a brief history of CSS, and also has links to their extensive CSS references and Learning Area, for people who want to know more.
This page explains that the browser uses the Document Object Model to render HTML elements, and to interact with them on the page.
This page gives a brief overview and history of the JavaScript language. It also provides links to tutorials that are made for various levels of experience.
JavaScript, alongside HTML and CSS, is the foundation of most websites. Manipulating the DOM through JavaScript is how you provide more functionality to previously static web pages.