Double-click the green flag after quitting from the pause menu. Arrow keys to move, z to jump/ homing attack. Down to slide. Down in the air to stomp. X to boost/ air boost. Enter key to open the pause menu. Hold A in debug mode to debug move. When you restart the level, the screen will flash black. This is intentional, it *is supposed to* keep you from seeing all the stuff that becomes extremely derpy during the restart.
Credit to piggeh for the Modern Sonic engine. Credit to MrLog for the original Sonic Engine. Color and background created by yours truly. Notice through testing: Bugs with collision detection, as Sonic might get stuck in a wall on occasions.