How could they have thought that back then?

In the 19th Century most people believed in white supremacy, including most of those who opposed slavery. Today believing in white supremacy is considered a major sign of ignorance. But we in our time are not smarter, or better, than our forbears, so why do we find it so easy to accept racial equality?

Which leads us to as, what practices that we approve of today will make our descendants scratch their heads and say "How could they have thought that way back then."