JSFiddle - React, Tailwind, and code Playground

HTML

<header class="closed"></header>
<div id="content"></div>

CSS

body {
    background: #f2f2f2;
}

#content {
    width: 960px;
    margin: 0 auto;
    background: #fff;
    height: 400px; /* for demo */
    -moz-box-shadow: 0 1px 5px #ccc;
    -webkit-box-shadow: 0 1px 5px #ccc;
    box-shadow: 0 1px 5px #ccc;
    border-bottom-left-radius: 15px;
    -moz-border-radius-bottomleft: 15px;
    -webkit-border-bottom-left-radius: 15px;
    border-bottom-right-radius: 15px;
    -moz-border-radius-bottomright: 15px;
    -webkit-border-bottom-right-radius: 15px;
}

.closed {
    /*background: #fff;*/
    width: 860px;
    margin: 0 auto;
    height: 0;
    border-top: 80px solid #fff;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    -moz-box-shadow: 0 1px 5px #ccc;
    -webkit-box-shadow: 0 1px 5px #ccc;
    box-shadow: 0 1px 5px #ccc;
}