Made small changes and adjustments in margins.
CSS:
body{
background: url(http://placehold.it/350x150&text=FooBar1) repeat;
}
#octagon {
width: 100px;
height: 71px;
background: red;
position: relative;
margin: 99px 10px 0px 100px;
}
#octagon:before {
content: "";
position: absolute;
top: -29px;
left: 0;
border-bottom: 29px solid red;
border-left: 29px solid red;
border-right: 29px solid transparent;
width: 42px;
height: 0;
}