docs: function after(element, content) {
    if (isString(content)) {
      return element.insertAdjacentHTML('afterEnd', content);
    } else {
      // when the second parameter of 'insertBefore' is 'null' the content is
      // appended to the end of the 'parentNode'.
      return element.parentNode.insertBefore(normalizeContent(content), element.nextSibling);
    }
  }