mshtml.IHTMLDocument2 document = (mshtml.IHTMLDocument2)webbrowser1.Document.DomDocument;
            IHTMLBodyElement body = (IHTMLBodyElement)document.body;
            IHTMLTxtRange searchRange = (IHTMLTxtRange)body.createTextRange();
            //......
            string find = searchRange.text.Substring(x, y);
           
            if (searchRange.findText(find, 0, 0))
            {
                searchRange.select();
            }