<div class="div"></div>
.div{ width:0; height:0; border-right:50px solid transparent; border-left:50px solid transparent; border-bottom:50px solid red; }
.div{ width:0; height:0; border-right:50px solid transparent; border-left:50px solid transparent; border-top:50px solid red; }
.div{ width:0; height:0; border-top:50px solid transparent; border-bottom:50px solid transparent; border-right:50px solid red; }
.div{ width:0; height:0; border-top:50px solid transparent; border-bottom:50px solid transparent; border-left:50px solid red; }