video铺满div

*{
   margin:0; padding:0;
}
html,
body,
#app{
   height: 100%;
   width: 100%;
   min-width: 1300px;
   margin: 0;
   overflow: hidden;
   /*font-family: "Helvetica Neue",Helvetica,"PingFang SC","Hiragino Sans GB","Microsoft YaHei","微软雅黑",Arial,sans-serif;*/
}

.video{
   width: 100%;
   height: 100%;
   object-fit: fill  // 重点
}

<div id="app">
   <video src="./background.mp4" class="video" autoplay="autoplay" muted="muted" preload loop>
      your browser does not support the video tag
   </video>
</div>