This is how I would do it.
.triangle-left {
width: 0;
height: 0;
border-top: 22px solid transparent;
border-bottom: 22px solid transparent;
border-right: 22px solid black;
position: relative;
}
.triangle-left:after {
content: '';
width: 0;
height: 0;
border-top: 21px solid transparent;
border-bottom: 21px solid transparent;
border-right: 21px solid #ffffdffffd;
position: absolute;
top: -21px;
left: 1px;
}
Here it is on JSFiddle.