Art Nouveau is classified by art historians as the first truly modern style: "Art Nouveau was the first self-conscious, internationally based attempt to transform visual culture through a commitment to the idea of the modern" (Greenhalgh, 18). Art Nouveau artists were the first to postulate that the tremendous changes of the Industrial Revolution and 19th Century would also change not only the style, but the nature of art. Art Nouveau is not Victorian, but represents a strong break with Victorian artistic traditions. (What we commonly call "Victorian style" is referred to by art historians as Second Empire or Baroque Revival.)
Art Deco is not just visually different from Art Nouveau; it also lacks the transformative nature of its predecessor. Art Deco reflects and imitates the visual landscape of modernism without attempting to change it.
Ironically, in French Art Nouveau is called Modern Style! (an English term).