Made customizable 404 page have a {STANDARD404} variable available to allow embedding the "default" 404 content.
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>{#advanced_dlg.code_title}</title>
<script type="text/javascript" src="../../tiny_mce_popup.js"></script>
<script type="text/javascript" src="js/source_editor.js"></script>
<base target="_self" />
</head>
<body onresize="resizeInputs();" style="display:none; overflow:hidden;">
<form name="source" onsubmit="saveContent();return false;" action="#">
<div style="float: left" class="title">{#advanced_dlg.code_title}</div>
<div id="wrapline" style="float: right">
<input type="checkbox" name="wraped" id="wraped" onclick="toggleWordWrap(this);" class="wordWrapCode" /><label for="wraped">{#advanced_dlg.code_wordwrap}</label>
</div>
<br style="clear: both" />
<textarea name="htmlSource" id="htmlSource" rows="15" cols="100" style="width: 100%; height: 100%; font-family: 'Courier New',Courier,monospace; font-size: 12px;" dir="ltr" wrap="off" class="mceFocus"></textarea>
<div class="mceActionPanel">
<div style="float: left">
<input type="submit" name="insert" value="{#update}" id="insert" />
</div>
<div style="float: right">
<input type="button" name="cancel" value="{#cancel}" onclick="tinyMCEPopup.close();" id="cancel" />
</div>
</div>
</form>
</body>
</html>