XHTML Validate, Dreamweaver tabbed Panels <li>

Hi,
I am trying my best to build a robust web site.  I'm a newbie using Dreamweaver.  I have built a page on my site using Dreamweaver's Tabbed Panel tool.  I then used Dreamweaver's Validate tool.  It returns the following error message for the two <li> lines of code.

The tag: <li> doesn't have an attribute: "tabindex" is currently active version [XHTML 1.0 transitional]

Can you let me know what needs changing?

Thanks

<div id="TabbedPanels1" class="TabbedPanels">
        <ul class="TabbedPanelsTabGroup">
          <li class="TabbedPanelsTab style1" tabindex="0">Form</li>
          <li class="TabbedPanelsTab" tabindex="0">ePTFE</li>
          <li class="TabbedPanelsTab" tabindex="0">Porous Plastic</li>
          <li class="TabbedPanelsTab" tabindex="0">Sintered Bronze</li>
          <li class="TabbedPanelsTab" tabindex="0">Sintered Ceramic</li>
          <li class="TabbedPanelsTab" tabindex="0">PolyMesh&#8482;</li>
          <li class="TabbedPanelsTab" tabindex="0">eCharge</li>
          <li class="TabbedPanelsTab" tabindex="0">Quadro&#8482;</li>
          <li class="TabbedPanelsTab" tabindex="0">Other</li>
        </ul>
        <div class="TabbedPanelsContentGroup">
          <div class="TabbedPanelsContent">

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