<div id="demo"></div>
#demo {
width: 0;
height: 0;
position: relative;
border-top: solid 50px transparent;
border-right: solid 50px black;
/* 黑色大三角形 */
border-bottom: solid 50px transparent;
}
#demo:after {
content: "";
width: 0;
height: 0;
position: absolute;
left: 3px;
top: -45px;
border-top: solid 45px transparent;
border-right: solid 45px white;
/* 白色小三角形 */
border-bottom: solid 45px transparent;
}