Does Lux's ability trigger when a spell is countered or when its target becomes invalid?

Ruling: Lux's ability triggers when you play a spell, regardless of whether the target becomes invalid. However, if the spell is countered, Lux will not trigger.

Sequence:

• When a spell's targets become illegal, the spell still resolves (ignoring impossible instructions), and Lux triggers
• When a spell is countered, the spell does not resolve, and Lux does not trigger

Nuances:

• A spell with invalid targets due to effects like Retreat still counts as resolving, just with some instructions ignored
• Countering a spell (like with Wind Wall) prevents resolution entirely, which prevents Lux from triggering