HTML俄罗斯方块小游戏代码，界面简洁，难度可选：简单、一般、困难，键盘操作，↑：变换、←：左移、→：右移、↓：加速。(HTML Tetris game code, simple interface, simple and difficulty optional: general difficulties, keyboard, arrow: transform, -: left, right, down to: acceleration.)