<div class="backgroundvideo">
<video preload="auto" autoplay playsinline loop muted>
 <source src="https://ваш-сайт.ru/video.mp4">
</video>
</div>

<style>
    @media (min-width: 250.98px)
    {
        .backgroundvideo video{
            height:100%;
        }
    }
    
    
      @media (min-width: 479.98px)
    {
        .backgroundvideo video{
            width:400px;
        }
    }
    
    
    
    @media (min-width: 639.98px) { 
    
        .backgroundvideo video{
   width: 500px;
}
}


@media (min-width: 959.98px) { 
    
    .backgroundvideo video{
   width: 800px;
}
}
    
   @media (min-width: 1199.98px) {
        
        .backgroundvideo video{
         width: 100%;
}
    
} 
    
    
</style>