Border-^^^

main.css

    content: "";
    display: block;
    position: absolute;
    bottom: -9px;
    left: 0;
    width: 100%;
    height: 9px;
    background: -webkit-linear-gradient(transparent 0%, transparent 0%), 
                -webkit-linear-gradient(135deg, #f9f9f9 33.33%, transparent 33.33%) 0 0%, 
    transparent -webkit-linear-gradient(45deg, #f9f9f9 33.33%, transparent 33.33%) 0 0%;
    background-size: 0px 100%, 9px 9px, 9px 9px;
    -webkit-transform: rotate(180deg);
    -moz-transform: rotate(180deg);
    -ms-transform: rotate(180deg);
    -o-transform: rotate(180deg);
    transform: rotate(180deg);