* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  html, body {
    width: 100%;
    height: 100%;
    background: #000;          /* 黒背景 */
    overflow: hidden;
    cursor: crosshair;
  }
  
  canvas {
    width: 100%;
    height: 100%;
    display: block;
  }