WebGPU actually generates the speech entirely in the browser. Web Speech is great too, but less practical if the model is complicated to set up and integrate with the speech API on the host.
You could do text to speech on a 1Mhz Apple //e using the 1 bit speaker back in the 80s (software automated mouth) and MacinTalk was built into the Mac in 1984. I know it’s built into both the Mac and iOS devices and run off line.
But I do see how cross platform browsers like Firefox would want a built in solution that doesn’t depend on the vendor.
Firefox on Windows is one such application that still uses SAPI. I don't know what uses does on other operating systems. Like, on Android, I imagine it uses whatever is the built-in OS TTS API, which likely goes through Google Cloud.
But anything that sounds at all natural, from any of the OS or browser vendors, is going through some cloud TTS API now.