SOURCE

<div></div>

/*记忆口诀：盒子宽高均为零，三面边框皆透明。 */
div:after{
    position: absolute;
    width: 0px;
    height: 0px;
    content: " ";
    border-right: 100px solid transparent;
    border-top: 100px solid rgb(255, 72, 0);
    border-left: 100px solid transparent;
    border-bottom: 100px solid transparent;
}