The use cases are mostly really specific. Rather than focus on "tips" like this, I think "best practice" guides are much more helpful. Actually thinking through questions like how much should I link my DOM to my CSS (e.g., #container > p vs p.class_name) is generally more helpful imo than specific tasks that you can google.

These tips are neat but are not going to make you a pro. What will make you a pro is understanding the box model, colors / gradients, and selector preference/scope.

Some good, some questionable.

Being pro in css however lies within writing readable, maintainable, well documented css and these tips focus on none of that and will not make you any more or less pro than you were before reading them :)

Same here, I've mainly learned from just guess and check. Any tutorials I've tried (admittedly not many) are too basic and haven't given me a good mental model of how to approach a problem with CSS.

A lot of these are actually bad practice, or things you should already know. Before you use any of them, you should research each of the rules separately to see how they work.

Code maintainability is #1 in css, and is the hardest thing to do. The main focus should be on that, and not trying to be clever or tricky.