This is about the interface objects for SpeechSynthesis and SpeechSynthesisVoice, not about the functionality itself which was already supported. These interfaces should be exposed according to spec. Feature detection using window.speechSynthesis was already possible, but this makes feature detection using SpeechSynthesisVoice.prototype possible. Note that SpeechSynthesisEvent, SpeechSynthesisErrorEvent and SpeechSynthesisUtterance interface objects were already exposed.
Samples: https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesis https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesisVoice