Good christ, calm down there, skippy.
The cap is 85%. You can never nullify the enemy's damage. What kind of broken ass game design would that be?
Answer: Oblivion (100% magic resistance, 100% chameleon, etc)

Anyway, fyi (including posters who refuted this answer) yes, the cap for magic resistance is 85%.
And yes, I think that applies to any and all types of magic. It wouldn't make any sense if it didn't, after all.
Resistance to a specific element is just that: resistance only for that element. The element is still magic, so the cap applies.
You will take much more damage on Master difficulty, of course, so 15% is still going to hurt (probably a lot).
Don't be fooled by the numbers that appear in menus, either. Armor shows higher and higher numbers but is still capped in the game itself. The higher numbers just don't do anything, that's all. I think magic resistance is the same but I haven't bothered taking mine so high (I'm a Breton, so I have a boost right from the start, anyway). In other words, don't think just because you see numbers adding up to 120% or more that you actually get that benefit in the game.