The vast majority of "unblocked" browser versions of Hill Climb Racing are unauthorized ports. They are hosted without the permission of Fingersoft. While playing them is rarely prosecuted, hosting them constitutes a violation of copyright law and intellectual property rights.
However, simplicity ends where physics begins. The game uses a realistic (and hilarious) ragdoll physics engine. Go too fast, and you will crash into a valley. Go too slow, and you won't make it up the next mountain. The balance between acceleration, momentum, and fuel conservation is the heart of the challenge. hill climb racing unblocked