div{
    height: 100%;
    text-align: center;
}

@media only screen and (max-width: 576px){
    iframe{
        display: block;
        width: 80vw;
        height: 50vh;
        margin-top: 30px;
        margin-left: auto;
        margin-right: auto;
    }
} 


/* Small devices (portrait tablets and large phones, 600px and up) */
@media only screen and (min-width: 576px) and (max-width: 767.995px) {
    iframe{
        display: block;
        width: 80vw;
        height: 50vh;
        margin-top: 30px;
        margin-left: auto;
        margin-right: auto;
    }
} 

/* Medium devices (landscape tablets, 768px and up) */
@media only screen and (min-width: 768px) and (max-width: 991.995px){
    iframe{
        display: block;
        width: 80vw;
        height: 50vh;
        margin-top: 30px;
        margin-left: auto;
        margin-right: auto;
    }
} 

/* Large devices (laptops/desktops, 992px and up) */
@media only screen and (min-width: 992px) and (max-width: 1199.995px){
    iframe{
        display: block;
        width: 70vw;
        height: 50vh;
        margin-top: 30px;
        margin-left: auto;
        margin-right: auto;
    }
} 

/* Extra large devices (large laptops and desktops, 1200px and up) */
@media only screen and (min-width: 1200px) {
    iframe{
        display: block;
        width: 40vw;
        height: 50vh;
        margin-top: 30px;
        margin-left: auto;
        margin-right: auto;
    }
}