Taurus represents classic beauty. With Taurus’s stability, beauty is timeless. Some things will always be considered beautiful. Biological beauty standards like the golden ratio would fall under Taurus. Taurus is an Earth sign, a sign that specializes in the physical plane. Taurus is specialized to please the physical senses, which could be where biological beauty standards come from.
Libra represents beauty trends. Libra is esoterically ruled by Uranus. Sometimes, in order to be charming to others, you have to change things up and keep people guessing. This is why fashion trends come and go. Once some trends had their time to shine, a new trend will slip in and replace the old trend because of its novelty. Change makes a lot of sense for Libra, for it is a dynamic air and cardinal sign.