JSFiddle - React, Tailwind, and code Playground

HTML

<header>
  <div id="over">
    z-index: 2 ... 
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  </div>
</header>

<main>
  <div id="under">
   z-index: 1 ....
   z-index: 1 ....
   z-index: 1 ....
   z-index: 1 ....
   z-index: 1 ....
   z-index: 1 ....
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  </div>
</main>

CSS

header, main {
  display: block;
  width: 100%;
}

html, body {
  margin; 0px;
}

header {
  position: fixed;
  height: 100px;
  top: 0px;
  left: 0px;
  z-index: 1;
}
  header > #over {
    position: absolute;
    z-index: 2;
    
    display: block;
    height: 100px;
    width: 100%;
    margin-top: 25px;
    padding: 25px;
    background: #88F;
  }

main {
  margin-top: 150px;
}
  main > #under {
    position: absolute;
    width: 80%;
    left: 10%;
    padding: 25px;
    margin-top: -85px;
    background: #F88;
    z-index: 1;
  }