The type or namespace name 'HtmlParser' could not be found (are you missing a using directive or an assembly reference?)
Скачал через nuget
Вставил пример из AngleSharp:
var source = @"
Error 404 (Not Found)!!1 404. That’s an error.
The requested URL /error was not found on this server. That’s all we know.";
// Create a new parser front-end (can be re-used)
var parser = new HtmlParser();
//Just get the DOM representation
var document = parser.Parse(source);
//Serialize it back to the console
Console.WriteLine(document.DocumentElement.OuterHtml);