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