html, body {
    margin: 0;
    height: 100%;
}
#canvas {
    width: 100%;
    height: 100%;
}
p {
    margin: 0;
    padding: 20px;
    position: absolute;
    line-height: 1.5em;
    font-size: 1.2em;
    font-family: Arial, Helvetica, sans-serif;
}
#crosshair {
    width: 6px;
    height: 6px;
    background-color: #ccc;
    border: 1px solid #000;
    position: absolute;
    top: calc(50% - 3px);
    left: calc(50% - 3px);
}