Inside the first CommonPollAnswerList ul, there is an opening <li> and then inside that there is another opening <li> then two closing </li> tags which is invalid:
<div class="CommonPollArea">
<h4 class="CommonPollQuestion">Why does this not work?</h4>
<div class="CommonPollContent">
<div class="CommonPollDescription">
<p>Why does the poll not work on IE9</p>
</div>
<div id="PollVoteArea" class="CommonPollSwitchedContent" style="display: block;">
<ul class="CommonPollAnswerList">
<li>
<table cellpadding="0" cellspacing="0" border="0">
<tr>
<td><input type="radio" name="PollVote" value="1"></td>
<td class="CommonPollAnswer">Because the poll has invalid HTML</td>
</tr>
</table>
<li>
<table cellpadding="0" cellspacing="0" border="0">
<tr>
<td><input type="radio" name="PollVote" value="2"></td>
<td class="CommonPollAnswer">Because IE is rubbish</td>
</tr>
</table>
</li>
</li>
</ul>
<input name="PollVoteCommand" type="submit" value="Vote">
</div>
<div id="PollResultsArea" class="CommonPollSwitchedContent" style="display: none;">
<ul class="CommonPollAnswerList">
<li>
<div class="CommonPollAnswer">Because the poll has invalid HTML (0%)
</div>
<div class="CommonPollResultGraph">
<img src="/Themes/generic/images/poll_a_start.gif" height="19" class="poll-graph-start">
<img src="/Themes/generic/images/poll_none_mid.gif" height="19" class="poll-graph-none" width="257">
<img src="/Themes/generic/images/poll_none_end.gif" height="19" class="poll-graph-none-end">
</div>
</li>
<li>
<div class="CommonPollAnswer">Because IE is rubbish (0%)
</div>
<div class="CommonPollResultGraph">
<img src="/Themes/generic/images/poll_a_start.gif" height="19" class="poll-graph-start">
<img src="/Themes/generic/images/poll_none_mid.gif" height="19" class="poll-graph-none" width="257">
<img src="/Themes/generic/images/poll_none_end.gif" height="19" class="poll-graph-none-end">
</div>
</li>
</ul>
</div>
</div>
<div class="CommonPollFooter">
<ul class="CommonPollFooterList">
<li><a href="#" onclick="TogglePollSwitchedAreas(this); return false;">View Results</a></li>
<li>Total Votes: 0</li>
<li><a href="/ControlPanel/Tools/Reports/PollVotingReport.aspx?pid=10083&atid=0" target="_blank">User Voting Report</a></li>
</ul>
</div>
</div>