<!DOCTYPE html>
<html>
<head>
<style>
body, html, div[contenteditable] {
    width: 100%;
    background-color: lavender;
    margin: 0;
}

iframe {
    width: 100px;
    height: 100px;
}

.fixed {
    width: 10px;
    height: 10px;
    position: fixed;
    background: red;
    bottom: 0;
    right: 0;
}
</style>
</head>
<body>
<div>
    <iframe srcdoc="<body contenteditable style='height: 100%; background: lightgreen;'>1. Change selection using arrow keys.</body>"></iframe>
    <iframe srcdoc="<body contenteditable style='height: 100%; background: lightgreen;'>2. Change selection using arrow keys.</body>"></iframe>
    <p>3. Scroll this iframe to the very bottom.</p>
</div>
<div class="fixed"></div>
<p style="position: absolute; top: 400px;">4. Click the button labeled 'Click me'.</p>
<script>
function handleClick() {
    [...document.querySelectorAll("iframe")].map(frame => frame.contentDocument.body.innerHTML = "")
}

function scrollDown() {
    scrollTo(0, 400);
}
</script>
</body>
</html>