JSFiddle - React, Tailwind, and code Playground

HTML

<div class="centre">
    Welcome to [REDACTED].<br />
    <form method="POST" action="post.php">
        <textarea name="paste" rows="50"></textarea><br />
        <select name="paste_type">
            <option value="perm">Permanent</option>
            <option value="self">Self Destructing</option>
        </select><br />
        <input type="submit" value="Paste" />
    </form>
</div>

CSS

* {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    padding: 0; 
    margin: 0; 
}

body {
    background-color: #0099cc;
    color: #ffffff;
    font-size: 180%;
    margin: 10px;
}

.centre {
    text-align: center;
}

textarea {  /* position is inherited */
    left: 0; 
    top: 0; 
    right: 0;
    bottom: 100px;  /* Actually the height of select and input */
    width: 100%;
}