XDocument

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

class Program
{
    static void Main()
    {
        var doc = new XDocument(new XElement("test", "data"));
        
        Console.WriteLine(doc);
    }
}

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

class Program
{
    static void Main()
    {
        var styleInstruction = new XProcessingInstruction("xml-stylesheet", "href='styles.css' type='text/css'");

        var docType = new XDocumentType("html", "-//W3C//DTD XHTML 1.0 Strict//EN", "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd", null);

        XNamespace ns = "http://www.w3.org/1999/xhtml";
        var root = new XElement(ns + "html", new XElement(ns + "head",
                        new XElement(ns + "title", "An XHTML page")), new XElement(ns + "body",
                        new XElement(ns + "p", "This is the content"))
                        );

        var doc = new XDocument(
             new XDeclaration("1.0", "utf-8", "no"), new XComment("Reference a stylesheet"), styleInstruction,
               docType, root);

        doc.Save("test.html");
        
        Console.WriteLine(doc);
    }
}