C# Online Compiler | .NET Fiddle

// @nuget: HtmlAgilityPack

using System;
using System.Xml;
using HtmlAgilityPack;
					
public class Program
{
	public static void Main()
	{
		var html =
        @"<body>
            <h1>This is <b>bold</b> heading</h1>
            <p>This is <u>underlined</u> paragraph</p>
        </body>";

        var htmlDoc = new HtmlDocument();
        htmlDoc.LoadHtml(html);

        var htmlBody = htmlDoc.DocumentNode.SelectSingleNode("//body");
		
		DisplayChildNodes(htmlBody);

       HtmlNode styleNode = HtmlNode.CreateNode("<style>.page{color:black;}</style>");
HtmlNode jsNode= HtmlNode.CreateNode("<script></script>");
htmlBody.AppendChild(styleNode);
htmlBody.AppendChild(jsNode);

        Console.WriteLine("\n****After child node appended****\n");

        DisplayChildNodes(htmlBody);
	}
	
	static void DisplayChildNodes(HtmlNode nodeElement)
    {
        HtmlNodeCollection childNodes = nodeElement.ChildNodes;

        foreach (var node in childNodes)
        {
            if (node.NodeType == HtmlNodeType.Element)
			{
				Console.WriteLine(node.Name);
			}
        }
    }
}

 
// @nuget: HtmlAgilityPack
​
using System;
using System.Xml;
using HtmlAgilityPack;
                    
public class Program
{
    public static void Main()
    {
        var html =
        @"<body>
            <h1>This is <b>bold</b> heading</h1>
            <p>This is <u>underlined</u> paragraph</p>
        </body>";
​
        var htmlDoc = new HtmlDocument();
        htmlDoc.LoadHtml(html);
​
        var htmlBody = htmlDoc.DocumentNode.SelectSingleNode("//body");
        
        DisplayChildNodes(htmlBody);
​
       HtmlNode styleNode = HtmlNode.CreateNode("<style>.page{color:black;}</style>");