Ето ти Element класа: Display(int depth) метода се вика рекурсивно, ако в съответния елемент има вложени други елементи (children.Count > 0).

namespace Composite.Element
{
    using System;
    using System.Collections.Generic;
    using System.Text;

    public class Element
    {
        private IList<Element> children;

        public Element(string type, params Element[] children)
        {
            this.Type = type;
            this.children = new List<Element>(children);
        }

        public string Type { get; private set; }

        public void Add(Element child)
        {
            if (child == null)
            {
                throw new ArgumentNullException();
            }

            this.children.Add(child);
        }

        public void Remove(Element child)
        {
            if (this.children.Contains(child))
            {
                this.children.Remove(child);
            }

            throw new ArgumentException();
        }

        public string Display(int depth)
        {
            string intend = new string(' ', depth);
            StringBuilder sb = new StringBuilder();
            sb.AppendLine(string.Format("{1}<{0}>", this.Type, intend));

            if (this.children.Count != 0)
            {
                foreach (var element in this.children)
                {
                    sb.Append(element.Display(depth + 2));
                }
            }

            sb.AppendLine(string.Format("{1}</{0}>", this.Type, intend));

            return sb.ToString();
        }
    }
}

Ето ти Main-a: добавил съм ред Console.WriteLine(html.Display(0)), за да ти се визуализира output-a на конзолата.

namespace DOMBuilder
{
    using System;
    using System.IO;
    using Composite.Element;

    public class Program
    {
        public static void Main()
        {
            Element html = new Element("html",
                                            new Element("head"),
                                            new Element("body",
                                                new Element("section",
                                                    new Element("h2"),
                                                    new Element("p"),
                                                    new Element("span")),
                                                new Element("footer")));

            Console.WriteLine(html.Display(0));
            File.WriteAllText("index.html", html.Display(0));
        }
    }
}