SOURCE

<div></div>

html,body {
            width: 100%;
            height: 100%;
            margin: 0;
            padding: 0;
}
div {
            width:0;
            height:0;
            border-width:100px;
            border-style: solid;
            border-color: red transparent transparent transparent; /*将右边、下边、左边变成透明*/
            margin: 0 auto; /*水平居中*/
            position: relative; /*脱离文档流*/
            top: 50%; /*偏移*/
            margin-top: -50px;
}