In 2006, I entered a web design class in a local private school. We "learned" to design websites in Dreamweaver, using tables for the layout. I remember once, the teacher told us that, in order to achieve a particular layout, we had to delete the doctype. It didn't strike me as stupid or even wrong. I was still very new to the web world back then.
We then had a single accessibility class with a guest teacher. He taught us accessibility guidelines, about the necessity to seperate content and design, and eventually told us about CSS Zen Garden to showcase how it was possible to achieve multiple layouts with a single content page. My friend and I looked at each other and had that silent "Waou" look.
This revelation led me to buy a book about CSS, and got me my first job. If it wasn't for CSSZG, I wouldn't have enjoyed web design as much as I did and turned it into my job for 4 years. And I'm sure this simple project has enlightened thousands of other web designers as well.