MDN HTML
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.
MDN CSS Page
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.
Why MDN is Important
Web Design and Development is constantly changing, which makes it important to have access to the most accurate, up-to-date, and verifiable information. MDN Web Docs is the primary source for everything related to HTML, CSS, and JavaScript.