通过父元素的childNodes来访问，
可以通过nodeType过滤掉非注释的子节点
<body>
<div style="display:none" class="HTML_TEMPLATE" id="photo_viewer_main"><!--<div class="utilityWindow03" id="photoViewer" style="z-index: 51;"><div class="contents" id="mainArea">
<div class="selectionView">
<div id="thumbnailContainer" class="closeButton">
<TMPL_IF NAME="loading">
<p>ロード中 </p>
</TMPL_IF>
</div>
</div>
<div class="formButtons01"><form action="" method=""><input type="button" value="閉じる" class="formBt02 closeButton"/></form></div>
</div></div>
--></div>
<script type="text/javascript">
alert(document.getElementById('photo_viewer_main').childNodes[0].nodeType == 8);
alert(document.getElementById('photo_viewer_main').childNodes[0].data);
</script>
</body>
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