Testing which headgear protect from headbutting. Judging by the fact that scuffed mining helmet doesn't protect you but blast visor does, I would guess that it is only checking for Melee Defense.
EDIT: SEE HEADGEAR TESTING FOR UP TO DATE LIST
n - abstract domino mask
y - alien museum mask
n - black silver visor
n - black strands
y - blast visor
y - blindshot visor
n - buried crown
y - chained museum mask
n - charred pimp hat
n - containment helmet
n - cracked containment helmet
y - crude skull mask
n - crushed glasses
y - dashing hat
n - devil mask
n - distorted wolfman mask
- Emerald Gift party hat
- entertainment visor
- examination glasses
- fake skull mask
- festival mask
n - festive knit hat
- fiery museum mask
- filtered HUD
- fireworks shades
- frigid scarf
- gallery visor
- gas mask
- ghoulish crown
y - giant paper mache head
- glimmering HUD
- gold framed spectacles
y - guard helm
- heavy containment helmet
- HKGames helmet
y - immersive visor
- improved visor
- infested containment helmet
- Integrated Rams helmet
- investigator's hat
- knit cap
- lab visor
- lieutenant's shades
- lucky cap
- Midgard cap
- Midgard hardhat
- mummy mask
- navigator's glasses
- nice shades
- noir retro hat
- nurse cap
- old spectacles
- optical HUD
- optical visor
- ordinary scarf
- owl's mask
- paper mask
- party mask
- passthrough visor
- pimp hat
- polysteel mask
- ratty top hat
- reactive mask
- rebreather mask
- red-lensed goggles
- reinforced surgical mask
- sailing cap
- scarred silver mask
n - scuffed mining helmet
- security visor
- shining glasses
- shining mask
- silk top hat
- silver mask
- slick shades
- smoky glass visor
- snowy Santa cap
y - Stainless Skulls helmet
- steel mask
- surgeon's mask
- the best helmet
- trendy Midgard cap
- twisted glasses
- twitch visor
- unlucky cap
- wolfman mask
y - wrapped museum mask