      calc(100% - 1px) calc(100% + 100px), -100px calc(100% - 1px),
            1px 100%;
        }
        100% {
          background-position: calc(100% + 100px) 1px,
            calc(100% - 1px) calc(100% + 100px), -100px calc(100% - 1px),
            1px 0px;
        }
      }
    </style>
  </head>

  <body>
    <div class="box horse_run">小马跑起来~</div>
  </body>
</html>
