Here is a quick little example I wrote, run this code in a browser and see how it works. From you description, I wasn't really 100% percent sure as to what you needed, so this example has you enter 3 basic values, you then click Generate. The script runs through and grabs the values from the form, then assembles a variable. Pretty straight-forward. If this isn't what you needed then just ask.
<html>
<head>
<script type="text/javascript">
<!--
function generateHTML(){
var firstName = document.getElementById('firstName').value;
var lastName = document.getElementById('lastName').value;
var newsLetter = document.getElementById('newsletter').value;
var html = "<html>\n<head>\n<title>User Information</title>\n</head>\n<body>\n";
html = html + "Thank you for submitting your information " + firstName + "!";
html = html + "<br>You entered that your last name was " + lastName;
if(newsLetter=='No'){
html = html + " and that you do not wish to receive a newsletter.\n";
} else {
html = html + " and that you would like to receive a newsletter.\n";
}
html = html + "</body>\n</html>\n";
document.getElementById('outputCode').innerHTML = html;
document.getElementById('outputExample').value = html;
}
//-->
</script>
</head>
<body>
<table width="100%" height="100%"><tr valign="middle"><td align="center">
<table>
<form name="userInfo">
<tr><td>Your First Name:</td><td><input type="text" name="firstName" id="firstName" /></td></tr>
<tr><td>Your Last Name:</td><td><input type="text" name="lastName" id="lastName" /></td></tr>
<tr><td>Would you like a newsletter?</td><td><select name="newsletter" id="newsletter"><option value="Yes" />Yes<option value="No" />No</select></td></tr>
<tr><td colspan="2" align="center"><input type="button" onClick="generateHTML();" value="Generate!" /></td></tr>
</form>
</table>
<table>
<tr>
<td width="50%"><fieldset><legend>Output Example</legend><div style="height: 185px; width: 100%;" id="outputCode"></div></fieldset></td>
<td width="50%"><fieldset><legend>Copy and Paste Code</legend><form><textarea rows="10" cols="50" id="outputExample"></textarea></form></fieldset></td>
</tr>
</table>
</td></tr></table>
</body>
</html>
Hope it helps!
edit: Oh, and please do excuse the nasty tables, I didn't feel like doing a full layout for this one. ;-)