Copy markup

copy-markup.html

<button onclick="copyMarkup()">Copy source!</button>
<script>
  function copyMarkup() {
    const box = document.createElement('textarea');
    box.style.position = 'absolute';
    box.style.left = '-10000px';
    box.setAttribute('cols', '1000');
    document.body.appendChild(box);

    box.value = document.documentElement.innerHTML;
    box.select();
    document.execCommand('copy');

    e.target.innerHTML = "Success!";
    setTimeout(() => document.body.removeChild(box), 100);
  }
</script>