CSS

Is only recommended to learn CSS after you are familiar with HTML.

Fundamentals

MDN is all you'll need. Read it from beginning to end: https://developer.mozilla.org/en-US/docs/Web/CSS.

Particularly, focus on:

1. The basic syntax, common attributes (color, font, border) and basic selectors.
2. The Box Model.
3. CSS Layouts (Flexbox and Grids as a minimum).
4. Responsive Design and media queries.

Related utilities:

• https://css-tricks.com/snippets/css/a-guide-to-flexbox/
• https://css-tricks.com/snippets/css/complete-guide-grid/

Popular libraries

Learn only after you learn the fundamentals:

• https://styled-components.com/.

Advanced

When you want to architect a big scope system:

• https://storybook.js.org/tutorials/design-systems-for-developers/.

Be aware of

But not necessarily learn them:

• https://d3js.org/.
• http://tachyons.io/ or https://tailwindcss.com/.