rad
The mathematicians' radian: 2π to the turn, built to talk to sine and cosine.
One rad is the angle whose arc measures one radius: the full turn holds 2π radians, about 6.2832. It is mathematics' natural unit — in it the trigonometric functions need no conversion factors — and CSS certified it along with the other three of the <angle> type.
Its moment is when the angle comes from a calculation, not an eye: values born of formulas with sin(), cos() and atan2(), or generated by tools that think in π. For angles chosen by hand, deg remains the mother tongue.
.ponteiro {
rotate: 0.7854rad; /* π/4 */
}