A 12-year-old girl has an ulcer 5 mm in diameter at the bottom of her oral cavity. The ulcer is surrounded with a bright red tissue that pales when pressed. An ulcer biopsy was performed. Microscopy revealed a tumor composed of numerous blood-filled cavities. The cavities are lined with a single layer of endothelial cells. Between them there is stroma, made up of loose connective tissue. What type of tumor can be characterized by such clinical and laboratory findings?