MDN Web Docs is Mozilla's documentation resource for web technologies such as HTML, CSS and JavaScript, with references, guides and tutorials for web developers.
W3Schools is a free web developer education site offering tutorials, references and an online editor for HTML, CSS, JavaScript, SQL, Python and other languages.
Overreacted is Dan Abramov's personal blog on React, JavaScript and software engineering, with in-depth, approachable essays by a former member of the React team.