MDN Web Docs
Mozilla
MDN Web Docs is a comprehensive resource for learning and referencing web technologies, including HTML, CSS, JavaScript, and Web APIs. It offers tutorials, guides, code examples, and debugging tips to help you build, test, and troubleshoot web applications.
More resources on HTML & CSS
The Odin Project: Foundations Path
The Odin Project provides a free and comprehensive, project-based curriculum for aspiring web developers. Its Foundations path introduces essential web development concepts, including HTML and CSS, and is highly recommended for beginners.
css-tricks.com
CSS-Tricks is a comprehensive frontend development resource offering tutorials, articles, and code snippets focused on HTML, CSS, and modern web design. It features in-depth guides, a CSS Almanac, and practical examples for building responsive layouts and interactive UI.
A Complete Guide to Flexbox
Illustrated CSS-Tricks reference to the Flexible Box Layout module, explaining every container and item property, including flex-direction, flex-wrap, justify-content, align-items, align-content, gap, order, flex-grow, flex-shrink and flex-basis, with diagrams and examples. Readers can build one-dimensional CSS layouts confidently.
What The Flexbox?!
Free 20-video course by Wes Bos on CSS Flexbox, covering axes, direction, wrapping, flex-grow, flex-shrink, flex-basis and alignment, followed by projects such as navigation bars, pricing grids and app layouts, so learners can build responsive layouts confidently.
MDN Web Docs
Mozilla's reference for every HTML element and attribute, with browser compatibility tables, accessibility notes, and worked examples. Use it to check exact semantics and supported behaviour instead of guessing, and as the authoritative answer when tutorials disagree.
Flexbox & Grid in 20 Minutes
Traversy Media screencast walking through the CSS flexbox model: container and item properties, main and cross axes, alignment, wrapping, and ordering. After it you can build navigation bars and card rows without floats or positioning hacks.