See this answer on why this is a bad idea.
The answer depends on whether you need to get this table server-side or client-side.
On the client, use jQuery:
var table = $('table.tn_result');
On the client, if you're using ASP.Net WebForms, it will probably be easiest to add runat="server"
and id
attributes on the table(s) you want to get. Then you can just get them in the code-behind like any other control:
var table = this.TableId; //not really required, but hopefully makes things clear
if(table.Attributes["class"].Contains("tn_result"))
{
// do something with the table
}