HtmlAgilityPack: how to create indented HTML?

No, and it's a "by design" choice. There is a big difference between XML (or XHTML, which is XML, not HTML) where - most of the times - whitespaces are no specific meaning, and HTML.

This is not a so minor improvement, as changing whitespaces can change the way some browsers render a given HTML chunk, especially malformed HTML (that is in general well handled by the library). And the Html Agility Pack was designed to keep the way the HTML is rendered, not to minimize the way the markup is written.

I'm not saying it's not feasible or plain impossible. Obviously you can convert to XML and voilà (and you could write an extension method to make this easier) but the rendered output may be different, in the general case.

Fast, Reliable, Pure C#, .NET Core compatible AngleSharp

You can parse it with AngleSharp which provides a way to auto indent:

var parser = new HtmlParser();
var document = parser.ParseDocument(text);
using (var writer = new StringWriter())
{
    document.ToHtml(writer, new PrettyMarkupFormatter
                            {
                                Indentation = "\t",
                                NewLine = "\n"
                            });
    var indentedText = writer.ToString();
}