body {
    margin-top: 40px;
}

body::before {
    content: "XS";
    color: red;
    font-size: 2rem;
    font-weight: bold;
    position: fixed;
    top: 0;
    right: 0;
}
.box {
    background-color: lightblue;
    border: 1px solid blue;
    min-height: 50px;
    font-size: 2rem;
}

@media (min-width: 576px){
    body::before{
        content: "SM";
    }
}

@media (min-width: 768px){
    body::before{
        content: "MD";
    }
}

@media (min-width: 992px){
    body::before{
        content: "LG";
    }
}

@media (min-width: 1200px){
    body::before{
        content: "XL";
    }
}
@media (min-width: 1400px){
    body::before{
        content: "XXL";
    }
}