style.css

body {
    font-family: Avenir, Helvetica, Arial, sans-serif;
}

.prose {
    max-width: 600px;
    margin: 0 auto;
}

.prose h1 {
    color: #333;
}

.prose h2 {
    color: #555;
}

.prose h3 {
    color: #777;
}

.model-viewer {
    position: relative;
    height: 480px;
    width: 640px;
    margin: 0 auto;
}

.model-viewer > .step {
    position: absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    background-color: #ccc;
    opacity: 0.5;
    z-index: 1;
}

.model-viewer > .step:not(.active) {
    display: none;
}