JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
    <div class="flag">
    </div>
</div>

CSS

.box {
    /* Set dimensions and color of containing box */
    width: 100px;
    height: 30px;
    background: #03e;
}

.flag {
    float: left;
    /* Set left border to control width and color of flag */
    border-left: 20px solid #fff;
    /* Set top and bottom border each to half of box height */
    border-top: 15px solid transparent;
    border-bottom: 15px solid transparent;
    width: 0;
    height: 0;
}