Identity
Quercetin is a flavonol — one of the most abundant and widely studied plant polyphenols — found in onions, apples, capers, berries and many other plants. In cosmetics it is used under the INCI name Quercetin (also sold as Quercetin Dihydrate, the same molecule as a hydrate) as a plant antioxidant. Two scope notes matter. It is heavily marketed as an ingestible supplement (outside this register's topical scope), and the register separately covers it as a senolytic partner in Dasatinib + Quercetin — an entirely different, non-topical use. This card is the topical cosmetic entry only.
Mechanism (as proposed)
Quercetin is a potent free-radical scavenger and anti-inflammatory in the lab — genuinely one of the stronger flavonoid antioxidants by in-vitro measures. The pharmacology is real and well studied. The problem is delivery: quercetin is poorly water-soluble, chemically unstable, poorly penetrating, and yellow-tinted — a difficult molecule to get into skin at a useful, stable concentration, which is why derivatised or encapsulated forms are often used instead of the raw flavonol.
Reading the evidence honestly
As with myricetin and taxifolin, the decisive gap is the missing study type: little controlled human topical data for any skin outcome. The cosmetic case rests on in-vitro antioxidant results, and "potent antioxidant in a dish" translates unreliably to visible skin benefit — made harder here by quercetin's poor stability and penetration. Hence the D grade despite an impressive laboratory profile. The much larger quercetin literature concerns ingestible use (metabolic, anti-inflammatory, and senolytic), which this topical register does not cover under this entry.