Q
A quarter of a millimeter — the unit Japanese typography brought into the standard.
One Q is a quarter of a millimeter: 1/40 of a centimeter, the smallest of the absolute units. The name comes from the kyū (級) of Japanese phototypesetting, where type sizes were always counted in quarter millimeters — and it is the only CSS unit spelled with a capital letter.
It is the right choice when a stylesheet composes for the Japanese graphic standard, where body text is 13Q or 16Q by trade convention. Outside it, it serves as certified curiosity: proof that the standard agreed to learn to count like Tokyo.
p {
font-size: 16Q; /* = 4mm */
}