Assuming nested tables don't have unique attributes ( id , class or anything else ) to get the required one via
doc.DocumentNode.SelectSingleNode("//table[@width='500']")
Does XPath prohibit using table several times in its path ?
foreach (HtmlNode table in doc.DocumentNode.SelectNodes("//table/tr/center/table"))
throws exception as SelectNodes returns null.
If so how to tackle parsing of html with nested tables without specific attributes with Agility Pack ?