A lot of the CSS properties documented on MDN (reference of all properties available [1]) come with interactive examples, for instance background-color and flex to pick two random samples. There are great introductions to CSS concepts in general on MDN as well (flexbox, grid).

Contributions to MDN are welcome. All you need is an account, and their reference reads more like documentation and less like an ad :)

[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference