Best attributes to craft into a shield?

TL;DR Personally, I'd put Health into Defensive slots, since max guard is computed from max health, and Strength into Utility slots, because, with the exception of guard, I care more about offense than defense. And then evaluate Fade-Touched Masterworks globally, see below.

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In a way, it doesn't matter. Traits of gear apply to the character, not the item, so you could buff attack on your shield and defense on your sword. This is particularly true of Fade-Touched Masterworks. Think of your character having 3 Masterwork slots total (one in the shield, one in the weapon, one in the armor). How do you want to utilize those 3 slots?

Best to answer this sort of question by considering your character's buffs globally, all stats considered, and then the limitations of the slots on the schematic for the gear. Shields mainly have Defensive slots, though some have Utility Metal, which is quite useful for a Warrior, and one even has Utility Leather.

The Defensive slots limit you to: Health, Stagger, Melee Def, Fire resist.

The Utility slots limit you to: Strength and Constitution

Strength gives two benefits: Attack and Guard damage
Constitution gives two benefits: Health and melee defense

Considering all of your characters buffs and stats, what do you need the most from the above? Are you a tanking character, where defense and health is more important than attack, or a damage dealer where the opposite is true?