Iirc I read somewhere (maybe the old mothership, maybe phyrexia) that in the early days the art style was anything goes. In other words: there was no world building. Along the way a rough guide book what things should look like came along. This guide was made by Pete Venters who oversaw the art to get one feel in a world, but with different styles. This second age was around ice age (at least rath block). It was a rough guide and allowed for individual styles while simultaneously have a cohesive world. How he did that is still impressive. Of course, wizards does what it is best at and “improved” the world book guide by making it too strict and individual styles got lost. This third era blended together with cgi/cai (or maybe the cai was a bit earlier).