JSFiddle - React, Tailwind, and code Playground

HTML

<header>
  <div class="box1"></div>
  <div class="box2"></div>
</header>

SCSS

body {
  background: #a3a3a3;
  padding: 0;
  margin: 0;
}

header {
  background: white;
  position: relative;
  width: 100%;
  height: 50px;
  
  .box2 {
    box-shadow: inset 0 10px 10px -10px gray;
    background: #e9e9e9;
    width: 100%;
    height: 50px;
    position: absolute;
    top: 50px;
    left: 0;
  }
  
}