h5打包出来的index.css中引入的https://cdn.dcloud.net.cn/img/shadow-grey.png作用是什么

打包完h5在主css文件index.css中发现的引入外部资源，请问这个东西的意义是什么？
文件目录：build/h5/static/index.css

body::after {  
    position: fixed;  
    content: '';  
    left: -1000px;  
    top: -1000px;  
    -webkit-animation: shadow-preload .1s;  
    -webkit-animation-delay: 3s;  
    animation: shadow-preload .1s;  
    animation-delay: 3s  
}  

@-webkit-keyframes shadow-preload {  
    0% {  
        background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png)  
    }  

    100% {  
        background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png)  
    }  
}  

@keyframes shadow-preload {  
    0% {  
        background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png)  
    }  

    100% {  
        background-image: url(https://cdn.dcloud.net.cn/img/shadow-grey.png)  
    }  
}