I've noticed that while you can mark a deck as Pauper, it won't check for legality, nor cards may be browsed for Pauper legality when you are building a deck.
Noticed that simply filtering for "common" won't do the trick, like if you have (for example) an uncommon
Ancestral Mask it won't appear there, but it's still legal in Pauper (because a common version exists).