Sites open one at a time in random order. Neighbouring open sites join into clusters, tracked with union–find. The page watches for the moment a single cluster first touches both top and bottom.
Tap to open a site yourself. Each run restarts after it spans. The same ?seed= always gives the same run.
LITSite percolation on the square lattice, pc ≈ 0.592746. Every 30 steps the open-site count and the cluster-size totals are checked, or the page throws.
AMBEROn a finite grid the measured p varies from run to run, around pc.