можно и так

#hexagon {

width: 100px;

height: 55px;

background: red;

position: relative;

}

#hexagon:before {

content: "";

position: absolute;

top: -25px;

left: 0;

width: 0;

height: 0;

border-left: 50px solid transparent;

border-right: 50px solid transparent;

border-bottom: 25px solid red;

}

#hexagon:after {

content: "";

position: absolute;

bottom: -25px;

left: 0;

width: 0;

height: 0;

border-left: 50px solid transparent;

border-right: 50px solid transparent;

border-top: 25px solid red;

}