It's not the only codec, but it's the only high quality codec mandated by the spec and supported by all the browsers.

G711 is also mandated by the spec but it's a low quality codec intended for speech with a fixed 8kHz sampling rate. There are a few other codecs supported by Chrome and Safari but not Firefox.

H264 is also supported though it's limited to the Constrained Baseline profile [0]. That said, I have been able to use an H264 stream encoded with a Main profile that still worked in Chrome, so it could just be a strong recommendation.

https://developer.mozilla.org/en-US/docs/Web/Media/Formats/W...