JSFiddle - React, Tailwind, and code Playground

HTML

<div class="original"> Hello
    <div class="original-inner">
        <iframe width=400 height=400 src=""></iframe>
        <div class="absolute">
          <iframe width=100 height=100 src=""></iframe>
        </div>
    </div>
    
</div>

CSS

div.absolute {
    width: 90%;
    height:90%;
    top:0;
    left:0;
    border: 5px solid #000;
    background-color: #F00;
    position: absolute;
    box-sizing: border-box;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    padding: 0px;
}
div.original {
    border: 5px solid #000;
    background-color: #0f0;
}
div.original-inner {
    width:500px;
}