You actually have to fake it with two triangles….
.container {
margin: 15px 30px;
width: 200px;
background: #fff;
border: 1px solid #a00;
position: relative;
min-height: 200px;
padding: 20px;
text-align: center;
color: #fff;
font: bold 1.5em/180px Helvetica, sans-serif;
text-shadow: 0 0 1px #000;
}
.container:after,
.container:before {
content: '';
display: block;
position: absolute;
left: 100%;
width: 0;
height: 0;
border-style: solid;
}
.container:after {
top: 10px;
border-color: transparent transparent transparent #fdd;
border-width: 10px;
}
.container:before {
top: 9px;
border-color: transparent transparent transparent #a00;
border-width: 11px;
}