JSFiddle - React, Tailwind, and code Playground

HTML

<div class="clearfix">
  <header>Header Flex</header>
  <nav>Navigation</nav>
  <main>
    <section>Chapter 1</section>
    <section>Chapter 2</section>
    <section>Chapter 3</section>
    <section>Chapter 4</section>
  </main>
  <footer>Footer</footer>
</div>

CSS

* {
  box-sizing: border-box;
}
div {
  display: flex;
  margin: 70px 10px 120px;
}
header{
  position: fixed;
  top: 0;
  left: 0;
  height: 70px;
  width: 100%;
  background-color: #ddd;
  text-align: center;
  z-index: 1;
}
nav {
  flex: 1 1 auto;
  height: 300px;
  margin: 0;
  border: 4px solid blue;
}
main {
  flex: 3 1 auto;
  border: 4px solid purple;
}
section {
  width: 100%;
  height: 150px;
  border: 4px solid green;
}

footer {
  position: fixed;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 70px;
  background-color: #aaa;
  text-align: center;
}