Just to be clear, are you really saying that speech with text to speech is server hosted and not on device for Windows?

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.