In ASP.NET you can get the HTML of any control by creating a new instance of StringWriter. Using this instance of StringWriter create a new instance of HtmlTextWriter and then rendering the control to this HtmlTextWriter by using RenderControl() method. This method is defined in Control class; therefore every control will have this method. After rendering the control get the HTML from the StringWriter using ToString() method.
Code(C#) is as below:
System.IO.StringWriter stringWriter = new System.IO.StringWriter();
HtmlTextWriter newHtmlWriter = new System.Web.UI.HtmlTextWriter(stringWriter);
string output = stringWriter.ToString();
If you are rendering a Datagrid or any other control that has a textbox with "runat='server'" tag then this will not work. Because for every textbow with "runat='server'" tag you need to have a "form" with "runat='server'" tag. So if your textbox in Datagrid is a read-only textbox then better replace it with "label" so that the HTML can be rendered correctly.