body {
}

.div-main {
    margin: 0;
    width:70vw;
}

.li-how-to-use {
    font-weight: bold;
}

.div-iframe {
    position: relative;
    width: 100%;
    max-width:640px;
    aspect-ratio: 16/9;
}

iframe {
    position:absolute;
    width: 100%;
    height: 100%;
    border:0;
}