精灵图 还原尺寸

做两倍图步骤

  //1.设置背景
  background: url(../img/sprites@2x.png) no-repeat;
  //2.还原背景图尺寸
  background-size: 400 * @unit 400 * @unit;
  //3.定位到精灵图
  background-position: -7 * @unit -228 * @unit;