Web development is a field of computer programming which involves the development of websites. It has been on the rise since the 1990s and is growing rapidly with the advent of new technologies and platforms such as smartphones and tablets. The demand for web developers is high on account of the increasing number of companies and startups that rely on the Internet.
CSS-Tricks is a huge website dedicated to CSS, the language used to style HTML documents, and its application in web development. You can find thousands of tutorials on CSS, from learning how to use CSS3 to creating your own custom CSS frameworks from scratch.
3. Mozilla Developer Network
5. Khan Academy