You can include HTML content. One possibility is encoding it in BASE64 as you have mentioned.
Another might be using CDATA
tags.
Example using CDATA
:
<xml>
<title>Your HTML title</title>
<htmlData><![CDATA[<html>
<head>
<script/>
</head>
<body>
Your HTML's body
</body>
</html>
]]>
</htmlData>
</xml>
Please note:
CDATA’s opening character sequence: <![CDATA[
CDATA’s closing character sequence: ]]>