JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">
    <div class="nested">Content</div>
</div>

CSS

.header {
    height: 70px;
    width: 100%;
    background-color: #ccc;
    position: fixed;
    left: 0;
    top: 0;
    display: block;
} 

.nested {
    display: block;
    position: relative;
    margin-top: 70px;
    height:100vh;
   background-color: #ddd;
}