I think it's different for everyone. That moment while you are beginner with CSS when something clicks. You just start to get it. That is your CSS "Ah-ha!" moment. You might not instantly become a CSS master, but from this point forward, there is no going back.
Because everyone enters the field of web design with different backgrounds and under different circumstances, I think the "Ah-ha!" moment is different for everyone. A true beginner with no web design experience at all might watch all the underlines on the links on their page disappear with a single CSS statement and be hooked. A seasoned veteran of web design coming from an environment where they were responsible for layout changes in complicated table-based systems might marvel at the ease of a CSS based structure and be hooked.
More likely though, it is something much smaller which causes the spark. It is one small little victory, your "Ah-ha!" that feels so good that you are hooked. (Sounds kind of like heroin when I say it like that, but you know what I mean).
If you have one, I am hoping that you will share! But for now, I can explain mine.
For me, it was a couple of concepts that felt like an unlocking of real power. It was a combination of these concepts that were my "Ah-ha!" moment.
Every page element is a box.
I can control the size and position of those boxes.
I can give those boxes background images.
Sounds simple, but those things still feel powerful for me today.
If any of you folks remember your "Ah-ha!" moment, feel free to share them by sending them in or commenting below. I'd love to do another post on this and share other people's moments.