To update on above answer by vencedor and remove the extra empty character encoding added to your header/footer images/text you can do something like this:
<html
xmlns:o='urn:schemas-microsoft-com:office:office'
xmlns:w='urn:schemas-microsoft-com:office:word'
xmlns='http://www.w3.org/TR/REC-html40'>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title></title>
<!--[if gte mso 9]><xml>
<w:WordDocument>
<w:View>Print</w:View>
<w:Zoom>100</w:Zoom>
</w:WordDocument>
</xml><![endif]-->
<style>
<!-- /* Style Definitions */
p.MsoHeader, li.MsoHeader, div.MsoHeader{
margin:0in;
margin-top:.0001pt;
mso-pagination:widow-orphan;
tab-stops:center 3.0in right 6.0in;
}
p.MsoFooter, li.MsoFooter, div.MsoFooter{
margin:0in 0in 1in 0in;
margin-bottom:.0001pt;
mso-pagination:widow-orphan;
tab-stops:center 3.0in right 6.0in;
}
.footer {
font-size: 9pt;
}
@page Section1{
size:8.5in 11.0in;
margin:0.5in 0.5in 0.5in 0.5in;
mso-header-margin:0.5in;
mso-header:h1;
mso-footer:f1;
mso-footer-margin:0.5in;
mso-paper-source:0;
}
div.Section1{
page:Section1;
}
table#hrdftrtbl{
margin:0in 0in 0in 9in;
}
-->
</style>
<style type="text/css" media="screen,print">
body {
font-family: "Calibri", "Verdana","HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif;
font-size:12pt;
}
pageBreak {
clear:all;
page-break-before:always;
mso-special-character:line-break;
}
</style>
</head>
<body style='tab-interval:.5in'>
<div class="Section1">
Something page 1
<div class="pageBreak"></div>
Something page 2
<div class="pageBreak"></div>
<table id='hrdftrtbl' border='1' cellspacing='0' cellpadding='0'>
<tr>
<td>
<div style='mso-element:header' id="h1" >
<p class="MsoHeader">
<table border="0" width="100%">
<tr>
<td>
YOUR_HEADER IMAGE/TEXT
</td>
</tr>
</table>
</p>
</div>
</td>
<td>
<div style='mso-element:footer' id="f1">
<p class="MsoFooter">
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center" class="footer">
YOUR_FOOTER_TEXT
<g:message code="offer.letter.page.label"/> <span style='mso-field-code: PAGE '></span> of <span style='mso-field-code: NUMPAGES '></span>
</td>
</tr>
</table>
</p>
</div>
</td>
</tr>
</table>
</div>
</body>
</html>
This now includes full css components for header and footer segments.
The trick that actually removes the extra encoding is declaring yet another table within the MsoFooter and MsoHeader sections.