When I use itextsharp to Convert Html To Pdf ,It's Wrong :Invalid Nested Tag Strong Found， Expected Closing Tag Img

using iTextSharp.text;
using iTextSharp.text.html.simpleparser;
using iTextSharp.text.pdf;
using System.Web.UI.HtmlControls;
...
protected void Button1_Click(Object sender,System.EventArgs e)
        {
            Response.ContentType = "application/pdf";
            Response.AddHeader("content-disposition", "attachment;filename=this.pdf");
            Response.Cache.SetCacheability(HttpCacheability.NoCache);
            StringWriter sw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(sw);
            HtmlForm frm = new HtmlForm();
            GridView1.AllowPaging = false;
            GridView1.Parent.Controls.Add(frm);
            frm.Controls.Add(GridView1);
            frm.RenderControl(hw);
            StringReader sr = new StringReader(sw.ToString());
            Document PDFdoc = new Document(PageSize.A4, 10.0F, 10.0F, 10.0F, 0.0F);
            iTextSharp.text.html.simpleparser.HTMLWorker htmlparser =   new iTextSharp.text.html.simpleparser.HTMLWorker(PDFdoc);
            PdfWriter.GetInstance(PDFdoc, Response.OutputStream);
            PDFdoc.Open();
            htmlparser.Parse(sr);
            PDFdoc.Close();
            Response.Write(PDFdoc);
            Response.End();
        }