Why the beat stays even
Every click is booked a moment ahead on your sound card's own clock, which counts samples, rather than played when a page timer fires. Page timers drift by a few milliseconds and stall when the browser is busy; the audio clock does not, so at 120 BPM the clicks are 0.5 seconds apart at the start and still 0.5 seconds apart ten minutes later, at 300 BPM too, and in a background tab.
Time signatures and subdivisions
Pick a time signature and the first beat of each bar gets a higher click (turn Accent beat one off for an even pulse). In 6/8 the fourth beat gets a lighter accent too, so it feels like two groups of three, and 7/8 falls as 2 + 2 + 3. Subdivision adds quieter clicks between the beats: eighths, triplets or sixteenths, which helps with a slow passage full of fast notes.
Setting the tempo
Type a number from 30 to 300, drag the slider, or use the minus and plus buttons; hold Shift to move by 10. Space starts and stops it, and the arrow keys change the tempo by 1. To match a song, press Tap beside Start in time with it and the tempo follows your taps. The Italian word under the number is the marking that tempo usually goes by.
Questions
Does the metronome keep going if I switch tabs?
Yes. The clicks are booked on the audio clock by a background worker, so they stay in time in another tab. The dots only move while you can see them.
What tempo is Allegro?
Here, 120 to 155 BPM. Andante is 76 to 107, Moderato 108 to 119, Vivace 156 to 175 and Presto 176 to 199. The markings are traditions rather than rules, and books disagree by a few BPM either way.
Why do the clicks lag behind the dots in my headphones?
Bluetooth headphones and speakers hold back all sound by a tenth of a second or more. The gaps between clicks stay even, so playing along by ear works; use wired headphones or the device's own speaker if you need sound and dots together.