* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
}

.col {
    width: 400px;
    height: 300px;
    border: 1px solid #000;
}

.main-flex {
    display: flex;
    justify-content: center; /* Y */
    align-items: center; /* X */
}