.chequerboard{

    background-image:
            linear-gradient(45deg, #ccc 25%, transparent 25%),
            linear-gradient(135deg, #ccc 25%, transparent 25%),
            linear-gradient(45deg, transparent 75%, #ccc 75%),
            linear-gradient(135deg, transparent 75%, #ccc 75%);
    background-size:25px 25px; /* Must be a square */
    background-position:0 0, 12.5px 0, 12.5px -12.5px, 0px 12.5px; /* Must be half of one side of the square */
}