body {
    text-align: center;
    background: #222;
    color: #fff;
    font-family: Arial;
}
canvas {
    border: 2px solid #555;
    cursor: pointer;
    background: 
        linear-gradient(#111 1px, transparent 1px),
        linear-gradient(90deg, #111 1px, transparent 1px);
    background-size: 10px 10px;
}
button {
    padding: 10px 20px;
    margin: 10px 5px;
    font-size: 16px;
    cursor: pointer;
    background: #4CAF50;
    color: white;
    border: none;
    border-radius: 5px;
}
#tests {
    margin-top: 30px;
    padding: 20px;
    background: #333;
    display: inline-block;
    text-align: left;
    border-radius: 5px;
}
.ok { color: #0f0; }
.fail { color: #f00; }