Visuals

I really like Julia Evans' visuals - below are some related to CSS or HTML:

• CSS units: https://wizardzines.com/comics/units/units.png
• A few CSS selectors: https://wizardzines.com/comics/selectors/selectors.png
• The box model: https://wizardzines.com/comics/box-model/box-model.png
• Flexbox basics: https://wizardzines.com/comics/flexbox-basics/flexbox-basics.png

Articles

A Complete Guide to Flexbox: https://css-tricks.com/snippets/css/a-guide-to-flexbox/

Beginner Concepts: How CSS Selectors Work: https://css-tricks.com/how-css-selectors-work/

Beautiful articles by Amelia Wattenberger. What I particularly like about these posts is how interactive they are:

• The CSS Cascade or, How browsers resolve competing CSS styles: https://wattenberger.com/blog/css-cascade
• What does 100% mean in CSS? https://wattenberger.com/blog/css-percents

Practice

CSS diner: https://flukeout.github.io/

Flexbox froggy: https://flexboxfroggy.com/