Related to the “port-wine stain”-type birthmark. Can be present at birth or develop before age 20, and sometimes grows significantly (like other naevii).
This birthmark doesn’t actually affect the epidermis on a histological level like many do - it’s just an accumulation of adipocytes directly under the skin, among the dermal collagen. Sometimes it’s encapsulated, sometimes it’s connected to subcutaneous fat, but either way, if it’s removed, it isn’t known to recur.
Interestingly, the first descriptions of this condition aren’t considered to have been written until 1921, even though this book (from 1909) both illustrates it and refers to it with the same name and presentation as it’s known today. However, the book doesn’t include a description of clinical etiology of the condition, despite including similar conditions, such as naevus pigmentosum and naevus pilaris.
A Practical Treatise on Diseases of the Skin. James Nevins Hyde, 1909.