Howrse
I picked up my early HTML and CSS skills through designing and coding presentations for players of Howrse, a popular horse simulation game.
Besides balancing the various features that players wanted, such as quotes, profile pictures, and links, there were also several technical challenges that stemmed from a constrictive sandbox environment; the Howrse interface allowed only very limited HTML and inline CSS. Because of these constraints, graphics artists had to be inventive in programming to capture behavior that would be one-line implementations on the open web, such as invisible scrollbars, horizontal scrolling, and box placement.
Overall, I learned how to better customize my visual designs towards what the client wanted. I also discovered new ways of looking at the tools I had. The sandbox might have been limited in its scope, but that just challenged me to keep experimenting and discover that what I wanted to accomplish was entirely within my grasp.
Example of horizontal scrolling and invisible scrollbars in pure HTML, accomplished with some smart margins and overflows.
A "scroll-style" layout.
A travel-themed presentation that changes images every time the page is refreshed.