public static void Main()
<h1>This is <b>bold</b> heading</h1>
<p>This is <u>underlined</u> paragraph</p>
<h2>This is <i>italic</i> heading</h2>
<h2>This is new heading</h2>
var htmlDoc = new HtmlDocument();
var node = htmlDoc.DocumentNode.SelectSingleNode("//body/h1");
HtmlNode sibling = node.NextSibling;
if(sibling.NodeType == HtmlNodeType.Element)
Console.WriteLine(sibling.OuterHtml);
sibling = sibling.NextSibling;