JSFiddle - React, Tailwind, and code Playground

HTML

<div class="metric good">
    
    
</div>
<p>This is magical because the box can change widths and the arrow is always there, centered, and on top.</p>

CSS

body {
    text-align: center;
    padding-top: 50px;
}

.metric {
    display: inline-block;
    position: relative;
    height: 150px;
    width: 50%;
    background: #69c;
    overflow: hidden;
}

.metric:after {
    position: absolute;
    top: 0;
    left: 50%;
    margin-left: -250px;
    content: '';
    background: white;
    width: 500px;
    height: 0;
    border: 250px solid white;
    border-top: none;
    border-bottom: 50px solid #69c;
    box-sizing: border-box;
}