HTML5中<menuitem>标签的使用与定义

<menu type="context" id="mymenu">
  <menuitem label="Refresh" onclick="window.location.reload();" icon="ico_reload.png">
  </menuitem>
  <menu label="Share on...">
    <menuitem label="Twitter" icon="ico_twitter.png"
    onclick="window.open(&#39;//twitter.com/intent/tweet?text=&#39;+window.location.href);">
    </menuitem>
    <menuitem label="Facebook" icon="ico_facebook.png"
    onclick="window.open(&#39;//facebook.com/sharer/sharer.php?u=&#39;+window.location.href);">
    </menuitem>
  </menu>
  <menuitem label="Email This Page"
  onclick="window.location=&#39;mailto:?body=&#39;+window.location.href;"></menuitem>
</menu>