The demo might only work in Chrome, but the API is just a standard REST API. Most people will be using this API on the server side to generate an MP3 which is streamed to the user.
Chrome actually has its own TTS engine (as does FF, Safari, etc), though the quality obviously isn't as good.
https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynth...
(I work for GCP)