style.css

@import url('https://fonts.googleapis.com/css2?family=Poppins:wght@300;500;600;700&display=swap');  

* {
    padding: 0;
    margin: 0;
    box-sizing: border-box;
    text-decoration: none;
}

a:link, a:visited, a:hover, a:active {
    color: black;
}

html {
    font-family: Poppins, Arial, Helvetica, sans-serif;
    font-size: 62.5%;
    background-color: #ccc;
}


main {
    display: flex;
    align-items: center;
    justify-content: center;
    padding-top: 8px;
}

.container {
    position: relative;
    background-color: #ddd;
    padding: 15px 20px;
    text-align: center;
    width: 500px;
}

.help {
    position: absolute;
    top: 15px;
    right: 10px;
    opacity: .5;
    transition-duration: .5s;
}

.help:hover {
    opacity: 1;
}

.qrcode {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

.qrcode__form {
    display: flex;
    align-items: stretch;
    margin-bottom: 10px;
    margin-top: 10px;
}

.qrcode__form input {
    flex-grow: 1;
    margin-right: 10px;
}

.qrcode__form button {
    height: 35px;
    width: 80px;
}

input {
    font-family: Poppins, Arial, Helvetica, sans-serif;
    padding-left: 10px;

    box-shadow: 3px 3px 5px #aaa;
    outline: none;
    border: 1px solid #aaa;
    border-radius: 5px;
}

input:hover {
    box-shadow: 3px 3px 8px #555;
    transition-duration: .5s;
}

.btn {
    background-color: #3B82F6;
    color: white;
    border-radius: 5px;
    border: none;
    cursor: pointer;
    box-shadow: 3px 3px 5px #aaa;
    font-family: Poppins, Arial, Helvetica, sans-serif;
}      

.btn:hover {
    background-color: rgb(38, 117, 38);
    box-shadow: 3px 3px 8px #555;
    transition-duration: .5s;
}   

.legend {
    margin-bottom: 10px;
    text-align: left;
    display: none;
}

.legend li {
    list-style-position: inside;
}

#help:target {
    display: block;
}

@media screen and (max-width:768px) {
    .container {
        width: 360px;
    }
    .qrcode__form {
        display: flex;
        flex-direction: column;
    }
    
    .qrcode__form input {
        margin-bottom: 10px;
        margin-right: 0;
        height: 35px;
    }
    
    .qrcode__form button {
        height: 35px;
        width: 100%;
    }
}