/* 透明图预览：棋盘格底，方便看真透明边缘 */
.checker {
  background-image:
    linear-gradient(45deg, #ccc 25%, transparent 25%),
    linear-gradient(-45deg, #ccc 25%, transparent 25%),
    linear-gradient(45deg, transparent 75%, #ccc 75%),
    linear-gradient(-45deg, transparent 75%, #ccc 75%);
  background-size: 16px 16px;
  background-position: 0 0, 0 8px, 8px -8px, -8px 0;
}