body, html {
    height: 100%;
    width: 100%;
    display: grid;
    place-items: center;
}

.mainDiv {
    width: 30%;
    height: 300px;
    border: 1px solid black;
    display: grid;
    place-items: center;
}

@media (max-width: 768px) {
    .mainDiv {
        width: 50%;
    }
}

@media (max-width: 480px) {
    .mainDiv {
        width: 80%;
    }
}