I mean their physical appearance. Both them are golden skinned mer.

This isn't easy to answer since we have only Almalexia to represent their race, but a good guess still would be welcome.

Their beliefs. Yes I mean that physically.

Originally, they were physically identical.

It seems their physical change occurred when they renounced their daedra patrons - Azura in particular - and adopted the Tribunal as their deities.

Now, the Altmer are definetily more pleasant to gaze upon - especially some of the women.

The Dunmer are deeply repulsive. Even Orcs look less repulsive.

I don't think they're repulsive at all. Frightening.. a bit. A bit unsettling.