JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"></div>

SCSS

$bg: url(http://files.delphinpro.ru/jsfiddle-pattern-dark.png) fixed;
$block-bg: white;

body {
    background: $bg;
    padding: 50px;
}
.block {
    height: 400px;
    background: $block-bg;
    position: relative;
}
.block:before,
.block:after {
    content: '';
    width: 20px;
    height: 20px;
    position: absolute;
    top: 0;
    background: $bg;
}
.block:before {
    border-radius: 0 0 100% 0;
    left: 0;
}
.block:after {
    border-radius: 0 0 0 100%;
    right: 0;
}