I have the Dorling-Kindersley Star Wars Visual Guide of the OT, which came out in 1997, and there it says the Emperor looked the way he did due to his lifelong immersion in the Dark Side.

After watching TPM I expected there to be a progression of his Dark Side corruption becoming more evident with each passing entry, and I was sorely disappointed when ROTS establishes that actually it was due to Mace Windu deflecting Force Lightning back at him. So stupid.

I recall the official SW website came up with some bullshit explanation that actually Palpatine was using the Force to project a non-corrupted looking version of himself. Yeah right.