E1 · A1 · D2 · G2
Standard four-string tuning: E1, A1, D2 and G2. Low strings are where most browser tuners give up, because a 41 Hz fundamental is both quiet and easily confused with its own octave. Fork detects down to 70 Hz directly and snaps the octave above back down for anything lower.
Tap a dot to hear the string. On a phone speaker the low notes are reproduced through their harmonics rather than their fundamental, which your ear reassembles into the right pitch.