Don't know if this is the correct way but I'm using this approach:

public class DummyController : Controller
{
}

public static class XhtmlStringExtensions
{
    public static string ToParsedHtmlString(this XhtmlString xhtmlstring)
    {
        if (xhtmlstring == null || xhtmlstring.IsEmpty || string.IsNullOrWhiteSpace(xhtmlstring.ToHtmlString()))
        {
            return string.Empty;
        }

        var routeData = new RouteData();
        routeData.Values.Add("controller", "a");

        var hh = new HtmlHelper(new ViewContext()
        {
            HttpContext = new HttpContextWrapper(HttpContext.Current),
            ViewData = new ViewDataDictionary(),
            TempData = new TempDataDictionary(),
            Controller = new DummyController(),
            RouteData = routeData
        }, new ViewPage());

        string result;

        using (var writer = new StringWriter())
        {
            hh.ViewContext.Writer = writer;
            hh.RenderXhtmlString(xhtmlstring);
            writer.Flush();
            result = writer.ToString();
        }

        return result;
    }
}