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