You need to install the Unity web player plugin before playing, but after that, you're good to go.
This is called the Green Demon Challenge!
The name refers to something that is typically considered a boon in Mario games: the 1-up mushroom. Basically, you find and activate a 1-up mushroom but don't grab it. You try to collect all eight red coins in a level, as well as the star, without letting a 1-up mushroom catch up to you.