Demo
css
#one {
border-top: 200px solid gray;
border-right: 100px solid transparent;
height: 0;
width: 30%;
float:left
}
#two {
border-bottom: 200px solid black;
border-left: 100px solid transparent;
height: 0;
width: 30%;
float:left;
margin-left:-100px;
}
#wrap {
width:400px;
margin:0 auto;
}