Create diagonal border of a cell

style.css

td {
    padding: 50px;
    border: 2px solid black;
    background-image: -webkit-gradient(linear, left top, right bottom, color-stop(49%, white), color-stop(50%, black), color-stop(51%, white));
    background-image: -webkit-linear-gradient(top left, white 49%, black 50%, white 51%);
    background-image: -o-linear-gradient(top left, white 49%, black 50%, white 51%);
    background-image: linear-gradient(to bottom right, white 49%, black 50%, white 51%);
}