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).<p>Contributions to MDN are welcome. All you need is an account, and their reference reads more like documentation and less like an ad :)<p>[1]: <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/Reference" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/CSS/Reference</a>
The author of this site is also the author of Bulma (<a href="https://bulma.io/" rel="nofollow">https://bulma.io/</a>) - my goto CSS framework. He's also created another similar site for HTML: <a href="https://htmlreference.io/" rel="nofollow">https://htmlreference.io/</a>
The first page I looked at, for align-content, says:<p>The remaing space is 150px<p><a href="https://cssreference.io/property/align-content/" rel="nofollow">https://cssreference.io/property/align-content/</a><p>Also, I lead a fairly ad-free life, so there's way too many ads for me to consider using it regularly - those fixed-on-the-screen ones are especially intrusive. A shame as it looks like a good idea.