style.css

body {
    text-align: center;
    margin: 40px auto;
    max-width: 650px;
    line-height: 1.5;
    font-size: 18px;
    color: #444;
    padding: 0 10px;
    font-family: Tahoma, Verdana, Segoe, sans-serif;
}

h1, h2, h3 {
    line-height: 1.2;
    margin: 0;
}

section {
    margin: 40px auto;
}

/* -------------------------------------------------------------------------- */

#steamid_converter {
    min-width: 300px;
    width: 50%;
    border: 1px solid #444;
    margin: 20px auto;
}

#steamid_converter input {
    border: 1px solid #444;
    margin: 20px auto;
    display: block;
    padding: 5px;
    width: 80%;
}

#steamid_converter ul {
    list-style-type: none;
    padding: 0;
    padding-bottom: 20px;
    margin: 0 auto;
    width: 90%;
}

.steamid {
    font-weight: bold;
    padding-left: 5px;
}

/* -------------------------------------------------------------------------- */

#validation input[type=text]:valid {
    color: green;
}

input:invalid {
    color: red;
    box-shadow: none;
}

#validation form {
    margin: 10px auto;
    padding: 10px;
}

#validation input {
    padding: 5px;
    border: 1px solid #d3d3d3;
}