Complex sentence: Very early printed books were sometimes produced with spaces left for rubrics and miniatures, or were given illuminated initials, or decorations in the margin, but the introduction of printing rapidly led to the decline of illumination.
Split into simpler sentences: Very early printed books were sometimes produced with spaces left for rubrics and miniatures, or were given illuminated initials, or decorations in the margin. But, the introduction of printing rapidly led to the decline of illumination.