Android:
https://developer.android.com/reference/android/speech/tts/UtteranceProgressListener.html#onRangeStart(java.lang.String,%20int,%20int,%20int)

Chrome:
c/b/speech/tts_chromeos*

Android has the ability (after O) for a caller of the tts api to receive callbacks when a range gets spoken. We can translate this to Chrome's notion of word level callbacks. This is useful for various accessibility services.