CSS (Cascading Style Sheets) is a language for describing the appearance of ... of the standard) and CSS best practices, you can find helpful resources here.  

A simple visualization of selectors in CSS. Choose the selector in the menu and the correctly selected items will appear on the right, with a brief description of the selector under the menu.

A frontend coding quiz and it’s not easy. The first section is on CSS.

A 6-chapter tutorial on the basics of CSS, with more chapters to come. Some good in-depth stuff here.

Harry Roberts has updated his CSS Guidelines. Previously just a GitHub repo, the Guidelines now have their own domain and have been expanded and improved. Harry has also implemented a “pay what...

Chrome Dev Relations guy Paul Lewis has created a quick reference for those interested in what is triggered on a page when a given CSS property changes. For example, some properties will trigger...

Bootstrap’s Mark Otto has put together his own HTML and CSS style guide with lots of tips including order of properties, order of HTML attributes, selectors, comments, and lots more.

A simple app to fiddle with the various flexbox features and see the results appear in real-time on the page along with the full code. What’s cool about this is that you can also edit the text in...