Octogono

#octagon {
    width: 100px;
    height: 100px;
    background: red;
    position: relative;
}

#octagon:before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    border-bottom: 29px solid red;
    border-left: 29px solid #eee;
    border-right: 29px solid #eee;
    width: 42px;
    height: 0;
}

#octagon:after {
    content: "";
    position: absolute;
    bottom: 0;
    left: 0;
    border-top: 29px solid red;
    border-left: 29px solid #eee;
    border-right: 29px solid #eee;
    width: 42px;
    height: 0;
}