Traditional etiquette states you do not say “Congratulations!” to the bride, rather you tell her “Best wishes!” and say congrats to the groom. The reason for saying congratulations to the groom is to tell him “good job” for landing a wonderful woman who agreed to marry him. The bride, in comparison, is wished a happy marriage. Saying congratulations to the bride (traditionally) implies that she “finally” found someone or it was difficult for her to obtain a husband.
If you’re interested, Jewish tradition states it is your job as a guest to make the happy couple, well, happy. This includes telling the bride how stunning she looks, which is guaranteed to make her smile and please her new husband.
Wish the bride in your life much love and many blessings, and don’t forget to tell her how amazing she looks! Congratulate the groom for asking such an incredible woman to marry him. Besides, if you have nothing nice to say to the bride, why are you attending the wedding in the first place? Do your part to make the day wonderful and memorable–the bride and groom will cherish your attendance, your gift, and your kind words. Have fun!