An online metronome is the version you do not install. You open the page, you set a number, you press Start, and the browser clicks. That is the whole product. There is no store listing, no account, and no project file. The click is a short tone from the Web Audio clock, scheduled a fraction of a second ahead so a busy page does not drag the tempo. The metronome home page explains that scheduling in more detail. This page is the one to bookmark when you only wanted the button.
The number in the box is beats per minute. 100, which is filled in here, is a click every 0.6 seconds. Change it before or during playback. The next gap uses the new tempo; clicks already handed to the clock are not rewritten. Beats per bar decides which click is higher. Leave it at 4 unless the music is in three or you want no accent at all, in which case set it to 1. The page does not click subdivisions and it does not swing. Doubling the BPM is the honest way to hear the offbeats.
Tap tempo is on the same row as Start. Tap with the record or with your foot, and the average gap becomes a BPM. It will not write a number outside 20 to 300. Start is still required, because a browser will not let a page make sound until you have clicked something. If the lamp flashes and you hear nothing, the tab is muted or the output device is wrong. If you hear nothing and the lamp is still, Start did not run, or the tempo field is not a number the page accepts.
Leave the tab in front while you play. Phones and laptops throttle pages you cannot see, and the timer that refills the schedule is part of the page. The audio clock itself is steady; the page has to keep feeding it. Stop ends the schedule. Closing the tab ends it too. Nothing about the tempo is stored. The BPM page is the same click with the number explained first, if you arrived wanting a definition rather than a start button.