Itisconsideredasthesymbolofpurity

for 后面的 it is considered as the symbol of purity 是 for 引导的原因状语从句。A bride usually wears white wedding dress in western wedding for it is considered as the symbol of purity.在西式婚礼上新娘通常身穿白色婚纱，因为它被认为是纯洁的象征。