JSFiddle - React, Tailwind, and code Playground

HTML

<div id="halfOctagon" >
</div>

CSS

#halfOctagon {
    width: 100px;
    height: 50px;
    background: #f35916;
    position: relative;
    top:25px;
    left:50px;
}

#halfOctagon:after {
    content: "";
    position: absolute;
    bottom: 0;
    left: 0;
    border-top: 29px solid #f35916;
    border-left: 29px solid #eee;
    border-right: 29px solid #eee;
    width: 42px;
    height: 0;
}