JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
    <div class="banner"></div>
    <iframe></iframe>
</div>

CSS

body {
    margin: 0;
}
.parent {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}
.parent .banner {
    background: #f00;
    width: 100%;
    height: 30px;
}
.parent iframe {
    background: #000;
    border: none;
    flex-grow: 1;
}