using System;

using AngleSharp.Dom;

using AngleSharp.Html.Parser;

public class Program

{

    public static void Main()

    {

        string html = "<div class=\"styles__Accordion___blue\"><h1>Hello</h1></div>";

        HtmlParser p = new HtmlParser();

        var f = p.ParseFragment(html, null);

        var e = f.QuerySelector("div[class^='styles__Accordion___']");

        Console.WriteLine(e.TextContent);

    }

}