On a pale cell the ant turns right, on a dark cell it turns left, then it flips the cell and steps forward. Its heading counts quarter-turns modulo 8, so its sign (+ or −) returns only after two full laps. It paints in its current sign.
Tap to add another ant. The same ?seed= always gives the same run.
LITLangton’s ant (1986); the highway appears after about 104 steps from a blank grid. The dark-cell count, and the rule heading = start + net turns (mod 8), are checked every 30 steps, or the page throws.
AMBERThe 720° sign is the site’s spinor bookkeeping; the ant’s path does not depend on it.