How can one learn modern CSS and is there an online reference that is up to date?

MDN is what you're looking for https://developer.mozilla.org/en-US/docs/Learn_web_developme...

There are a number of sites that aggregate some things, here is one

https://moderncss.dev/topics/

Recent survey of what people use to learn CSS:

https://2025.stateofcss.com/en-US/resources/

CSS Tricks article on this:

https://css-tricks.com/how-to-keep-up-with-new-css-features/