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It is in Blink and Gecko: https://chromium.googlesource.com/chromium/src/+/master/third_party/WebKit/Source/modules/speech/SpeechSynthesisUtterance.idl https://github.com/mozilla/gecko-dev/blob/master/dom/webidl/SpeechSynthesisUtterance.webidl
Suggested errata: "Section 5.2 Make SpeechSynthesisUtterance's voice attribute nullable to match implementations"
https://github.com/w3c/speech-api/pull/22