JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header"></div>
<div id="side"></div>
<div id="body"></div>

CSS

#header {
    position: fixed;
    z-index: 2;
    left: 0%;
    top: 0%;
    width: 100%;
    height: 10%;
    background-color: purple;
}
#side {
    position: fixed;
    z-index: 2;
    left: 0%;
    top: 10%;
    width: 10%;
    height: 100%;
    background-color: red;
}
#body {
    position: absolute;
    left: 10%;
    top: 10%;
    width: 90%;
    height: 300%;
    background-color: orange;
}