Birds
Ratites
Fowl
Neoaves
Mammals
Monotremes
Echidnas
Platypus
Marsupials
Opossums
Numbats, etc.
Bandicoots, etc.
Kangaroos, koalas, wombats, etc.
Placentals
Primates, ungulates, etc.
Anteaters, sloths, etc.
Elephants, etc.
Reptiles
Lizards and snakes
Tortoises and turtles
Crocodilians
Tuatara
Amphibians
Frogs and toads
Salamanders and newts
Caecilians
A simple multi-level CSS dropdown menu.