JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myHeader">MySite</div>
<div id="myWrapper"><iframe id="myIframe" src="data:text/html;charset=utf-8,<div style='position:absolute;height:100%;width:100%;background-color:red;'>other Site</div>"></iframe></div>

CSS

body {
    background-color: black;
    margin: 0 0 0 0;
}

#myHeader {
    position: fixed;
    top: 0;
    height: 100px;
    width: 100%;
    background-color:blue;
}

#myWrapper {
    position: absolute;
    top: 100px;
    bottom: 0;
    left: 0;
    right: 0;
    background-color: yellow;
}

#myIframe {
    height: 100%;
    width: 100%;
    
}