CSS: triangles
.box {
background-color: red;
width: 200px;
height: 100px;
margin-top: 30px;
position: relative;
}
.box:before {
content: '';
position: absolute;
display: block;
left: 0;
top: -20px;
width: 0;
height: 0;
border-width: 10px;
border-color: red green blue yellow;
border-style: solid;
}