Markup block - Gen<None> - 9 - (0:0,0)
    Markup span - Gen<Markup> - [<!--<foo>] - SpanEditHandler;Accepts:None - (0:0,0) - Symbols:6
        HtmlSymbolType.OpenAngle;[<];
        HtmlSymbolType.Bang;[!];
        HtmlSymbolType.DoubleHyphen;[--];
        HtmlSymbolType.OpenAngle;[<];
        HtmlSymbolType.Text;[foo];
        HtmlSymbolType.CloseAngle;[>];
