<div id="arrow"></div>

#arrow {
	@unitSize: 20px;
	width: 0;
	height: 0;
	border-top: @unitSize solid transparent;
	border-left: (@unitSize * 2) solid red;
	border-bottom: @unitSize solid transparent;
	margin-left:  @unitSize;
	
	&:before {
		content: "";
		position: absolute;
		width: @unitSize ;
		height: @unitSize / 2;
		background: red;
		left: 0;
		top: @unitSize / 1.4;
	}
}