I've found Ariel Flesler's XMLWriter constructor function to be a good start for creating XML from scratch, take a look at this
http://flesler.blogspot.com/2008/03/xmlwriter-for-javascript.html
Example
function test(){
var v = new XMLWriter();
v.writeStartDocument(true);
v.writeElementString('test','Hello World');
v.writeAttributeString('foo','bar');
v.writeEndDocument();
console.log( v.flush() );
}
Result
<?xml version="1.0" encoding="ISO-8859-1" standalone="true" ?>
<test foo="bar">Hello World</test>
One caveat to keep in mind is that it doesn't escape strings.
See also
Libraries to write xml with JavaScript
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