css 三角形

#demo {

width: 100px;

height: 100px;

background-color: #333;

position: relative;

}

#demo:after {

border: solid transparent;

border-left-color: #333;

border-width: 10px;

width: 0;

content: " ";

position: absolute;

left: 100%;

top: 10%;

}

#demo:before{

border: solid transparent;

    border-right-color: red;

    border-width: 10px;

    width: 0px;

    content: " ";

    position: absolute;

    right: 100%;

    top: 70%;

}