.mainvid{
    width: 600px;
    height:  450px;
    
}
@media screen and (min-width: 576px) and (max-width: 820px) { 
    .mainvid{
        width: 500px;
        height:  350px;
    }
}

@media screen and (min-width: 280px) and (max-width: 576px) { 
    .mainvid{
        width: 270px;
        height:  200px;
    }
}