.container{
    position: relative;
    width: 200px;
    height: 200px;
    background: #abcdef;
    -webkit-border-radius: 20px;
    -moz-border-radius: 20px;
    border-radius: 20px;
}
.triangle{
    position: absolute;
    top: 20px;
    right: -20px;
    width: 0;
    height: 0;
    line-height: 0;
    background: #000;
    border-left: 10px solid #abcdef;
    border-right: 10px solid #fff;
    border-top: 10px solid #fff;
    border-bottom: 10px solid #fff;
    overflow: hidden;
}

<!-- html -->
<div class="container">
    <div class="triangle"></div>
</div>