Structure flexible markup tag

structure-entries.html

<ul id="nav">
  {exp:structure:entries}
    <li class="top-level" id="{page_slug}"><a href="{page_url}">{title}</a>
      {if children}
        <ul class="dropdown">
          {children limit_depth="2"}
            <li class="level-{depth}"><a href="{page_url}">{title}</a>
          {/children}
        </ul>
      {/if}
    </li>
  {/exp:structure:entries}
</ul>