Sorry if this doesnt belong here, but I figured this would be the best place. I know they are playable under some restrictions, but what exactaly are the restrictions? Mana cost must be visable? Name? I'm just unsure, thanks
So basically the card has to be recognizable, have the same size and weight, not convey any information that you could use to cheat, and beyond that it is up to the judge.
Common guidelines for alters are:
Mana cost visible
Name and type at least partially visible
Rules text does not have to be visible. In fact, wizards has made full arts of their own..from {Island} to {Day of Judgement}.
To my knowledge it is ultimately up to the judge to determine legality of an alter.
You're the best, thanks