If a spell targets something 'at a battlefield' and the target moves to a different battlefield before the spell resolves, is the target still legal?

Ruling: Yes, the target is still legal. The game only checks if the target meets the targeting requirements when the spell resolves - it needs to be at a battlefield, not necessarily the same battlefield you originally targeted.

Nuances:

• Spells don't "fizzle" even if they have illegal targets - they resolve to no effect, which matters for "when you play a spell" triggers
• If the target moves to base (not at any battlefield), then it would no longer be a legal target