<head>
<meta name="viewport" content="width=device-width">
<style>
body {
    width: 100%;
    height: 100%;
    margin: 0;
}

#source, #destination {
    width: 100%;
    height: 200px;
    font-size: 200px;
    white-space: nowrap;
}

#destination {
    border: black 1px solid;
}
</style>
</head>
<body>
<div id="source">Hello world</div>
<textarea id="destination"></textarea>
<script>
let text = source.childNodes[0];
getSelection().setBaseAndExtent(text, 0, text, text.data.length);
</script>
</body>