triangle

HTML

<div class="cover">
<div class="arrow-left"></div>
</div>

CSS

.arrow-left {
    position:absolute;
    width:100px;
    height:100px;
    left:20px;
    background:black;
    -webkit-transform:rotate(45deg);
    transform:rotate(45deg);
    border-radius:10px;
}
.cover {
position:absolute;
height:100px;
width:40px;
overflow:hidden;
}