JSFiddle - React, Tailwind, and code Playground

HTML

<body>
        <div id="container">
            <div id="header">
                START OF HEADER CONTENT and the viewer should be able to see all of it. Ipsum Blaster jango fett alderaan data dooku validium hypercube. Antilles mace windu uhura xindi millenium falcon bothan exterminate tylium ore. Endor maul skywalker everlasting matches antilles FTL tylium ore dooku. Paradox machine padawan speeder doctor who chewbacca AT-AT frack dagobah ice gun. Uhura data photon torpedo worf landspeeder starbuck hypercube cantina. END OF HEADER CONTENT
            </div>
            <div id="main">
Craft beer jean shorts beard green juice, kinfolk 8-bit hoodie single-origin coffee letterpress seitan four dollar toast hammock occupy selfies pug. Retro locavore meditation craft beer viral, vinyl health goth cred butcher echo park. Echo park portland helvetica roof party hammock, food truck messenger bag pop-up poutine master cleanse hella artisan etsy. Celiac polaroid before they sold out tousled chillwave farm-to-table, leggings craft beer mlkshk viral seitan vegan intelligentsia. Yuccie synth poutine next level food truck, meggings chambray aesthetic farm-to-table marfa helvetica dreamcatcher blue bottle mumblecore brunch. Fixie narwhal lomo, art party pinterest direct trade poutine mlkshk organic forage irony. Wolf kickstarter authentic dreamcatcher plaid.

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Four dollar toast sriracha hammock...

CSS

#container {
    float: left;
    height: 100%;
    background-color:gray;
    position: relative;
}

#header {
    background-color: green;
}

#main {
    overflow: auto;
    background-color: red;
    height: 70%;
}

#footer {
    background-color:yellow;
}