select2/docs/_includes/options-new/data/select.html

<section>
  <h2 id="data-adapters-select-tag">
    Can Select2 be used with a <code>&lt;select&gt;</code> tag?
  </h2>

  <h3>
    Does Select2 support nesting options?
  </h3>

  <p>
    Yes, just like in a standard <code>select</code>.
  </p>

  <h3>
    How many levels of nesting can there be?
  </h3>

  <p>
    Only a single level of nesting is allowed per the HTML specification.
  </p>

  <h3>
    Can <code>&lt;optgroup&gt;</code> tags be made selectable?
  </h3>

  <p>
    No.
  </p>

  <h3>
    How are <code>&lt;option&gt;</code> and <code>&lt;optgroup&gt;</code> tags serialized into data objects?
  </h3>
</section>