Clock Chimes runs in the background to play quarter and hour chimes accurately.

It uses a web worker that runs in an operating system thread, independent of the browser.

The chime sounds are synthesized, keeping the document size small.

It works in Gecko (Firefox), Blink (Chromium, Edge), QtWebEngine (Falkon, qutebrowser), and WebKit (Epiphany, Midori), but the latter will not load the embedded tick-tock sound files.  QtWebEngine’s audio sometimes lags in the background.  Each chime is logged below, as seconds and milliseconds, so you can see when the script actually told the browser to play it.