Whether you are looking to learn how to code or just looking for reference sites, here's a collection of online resources you can use for free online. From degree and certificate courses to reference sites, its never been easier to learn software development and this should make it even easier. Suggest anything we might have missed at the form linked below. Or on the linked telegram group.
Your Mastery starts here. Every website relies on HTML, almost every website uses CSS to look just right.
MDN Web Docs
developer.mozilla.org
The MDN Web Docs site provides information about Open Web technologies including HTML, CSS, and APIs for both Web sites and progressive web apps. It also has some developer-oriented documentation for Mozilla products, such as Firefox Developer Tools.