<label for="html-selector">Выберите страницу:</label>
<select id="html-selector">
  <option value="">-- Выберите --</option>
  <option value="/wp-content/custom-html/page1.html">Страница 1</option>
  <option value="/wp-content/custom-html/page2.html">Страница 2</option>
</select>

<div id="html-content" style="margin-top: 20px; border: 1px solid #ccc; padding: 20px;"></div>

<script>
document.getElementById('html-selector').addEventListener('change', function () {
  const url = this.value;
  if (!url) return;
  fetch(url)
    .then(r => {
      if (!r.ok) throw new Error("Ошибка загрузки HTML");
      return r.text();
    })
    .then(html => {
      document.getElementById('html-content').innerHTML = html;
    })
    .catch(err => {
      document.getElementById('html-content').innerHTML = `<p style="color:red;">${err.message}</p>`;
    });
});
</script>