This is the desired shape intended to be achieved using CSS only. I think it can be done using borders CSS property, but could not make it happen.
One option is:
http://jsfiddle.net/j08691/a0uogod0/
#arrow {
border-right:30px solid black;
border-bottom:30px solid black;
width:100px;
height:100px;
transform: rotate(-45deg);
margin-top:40px;
}
JSFiddle accredited to user j08691 from How to Make A Fancy Arrow Using CSS?