<!-- videoBG -->
  <video playsinline autoplay muted loop poster="polina.jpg" id="bgvid">
 
    <source src="polina.webm" type="video/webm">
  
    <source src="./picture/back.mp4" type="video/mp4">
  
  </video>
  <div id="bg-over"></div>
 <style>
  video#bgvid { 
    position: fixed;
    top: 50%;
    left: 50%;
    min-width: 100%;
    min-height: 100%;
    width: auto;
    height: auto;
    z-index: -100;
    -ms-transform: translateX(-50%) translateY(-50%);
    -moz-transform: translateX(-50%) translateY(-50%);
    -webkit-transform: translateX(-50%) translateY(-50%);
    transform: translateX(-50%) translateY(-50%);
    background: url(polina.jpg) no-repeat;
    background-size: cover; 
}
</style>

可以引用也可以直接放到本体中。