Identity
Myricetin is a plant flavonol — a polyphenol in the same family as quercetin and kaempferol, found in berries, tea, grapes and many vegetables. Its chemical name, hexahydroxyflavone, flags what makes it a strong antioxidant: six hydroxyl groups, plenty of sites to donate electrons and quench free radicals. In cosmetics it is used under the INCI name Myricetin as a plant-flavonoid antioxidant. (It is also heavily marketed as an ingestible supplement — that use sits outside this register's topical scope.)
Mechanism (as proposed)
As an antioxidant, myricetin scavenges reactive oxygen species generated by UV and pollution, and in the lab it also shows anti-glycation activity (interfering with the sugar-protein cross-linking implicated in skin ageing) and some UV-protective effects in cell and model systems. The in-vitro pharmacology is genuinely strong — which is exactly the trap this register watches for. Two physical limits temper it on skin: myricetin has poor stability and limited penetration, and like other flavonols it is prone to oxidising and discolouring in a formula, which constrains both delivery and shelf life.
Reading the evidence honestly
The decisive fact is the absence of the right kind of study: there is little to no controlled human topical data on myricetin for any skin outcome. The cosmetic case is built almost entirely on in-vitro antioxidant and anti-glycation results, and "potent antioxidant in a dish" has a long, well-documented record of not translating to visible human benefit. That is why the grade sits at D despite a legitimately impressive laboratory profile. Where myricetin does have a larger literature — metabolic and other effects — it concerns ingestible use, which this topical register does not cover.