JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div id="box"></div>
    <div>

CSS

.container {
    width: 100%;
    overflow: hidden;
}
#box {
    background-color: red;
    position: relative;
    height: 150px;
}
#box:before {
    position: absolute;
    content:'';
    border-bottom: 150px solid white;
    border-left: 150px solid transparent;
    border-right: 150px solid transparent;
    left: -150px;
    bottom: 0;
}