Hola estuve experimentado y logre esto:
Código HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<style type="text/css">
.triangle{
border-color: #f00 transparent transparent transparent;
border-style: solid;
border-width: 50px;
height:0;
width:0;
top:100px;
position:absolute;
z-index:1;
}
.rectangulo{
background-color: #F00;
height:100px;
width:30px;
position:absolute;
left:43px;
}
</style>
</head>
<body><div id="flecha">
<div class="rectangulo"></div>
<div class="triangle"></div>
</div>
</body>
</html>