public static IDisposable Tag(this HtmlHelper html, string tagName)
{
    if (html == null)
        throw new ArgumentNullException("html");

    Action<string> a = tag => html.Write(String.Format(tag, tagName));
    a("<{0}>");
    return new Memento(() => a("</{0}>"));
}

// Used like

using (Html.Tag("ul"))
{
    this.Model.ForEach(item => using(Html.Tag("li")) Html.Write(item));
    using(Html.Tag("li")) Html.Write("new");
}