The Mystery Room
The Main Interface
You can't tell from looking at this, but the background stars were animated and multilayered, using CSS3. All instructions were given in the form of specially prepared youtube clips. I particularly liked the way the numbers scrolled when you clicked them, though the effect was lost a bit on slower tablets.
The main puzzle on the site was like one of those old make-your-own-adventure books. You watch a clip, then make a choice. The next clip you get depends on that choice. As you can imagine, this very quickly results in a lot of youtube clips.
I really went to town on the ending. A star wars screen scroller coupled with a user-controllable fireworks display. If that wasn't enough, Cliff Richard sings congratulations. click here to see it in effect.