styles.css

*{
    box-sizing: border-box;
}
body{
    background: #7cddfaa9;
    font-family: sans-serif;
    font-weight: bold;
    max-width: 500px;
    height: 100vh;
    margin: 0 auto;
    display: flex;
    justify-content: center;

    align-items: center;
    color: #fff;
}
h1{
    display: flex;
    font-weight: bold;
    color: #000000;
    justify-content: center;
    font-size: 70px;
}

.qr-text{
    padding: 1rem;
    display: block;
    background: #ffffff;
    border: 1px solid #4e5b96;
    border-radius: 5px;
    width: 100%;
    margin: 0.5rem 0 1rem 0;
    color: #000000;
    outline: none;
}
.qr-text::placeholder{
    color: #000000ad;
    font-weight: 500;
}
#qr-code{
    height: 500px;
    width: 500px;
    display: flex;
    justify-content: center;
}

@media (max-width: 512px){
    body{
        max-width: 100%;
        padding: 10px;
    }
    #qr-code{
        width: 95vw;
        height: 95vw;
    }
    #qr-code img, #qr-code canvas{
        max-width: 95vw !important;
        max-height: 95vw !important;
    }
}