What do break, emphasis and prosody mean in W3C SSML 1.1, and how much is a TTS engine actually obliged to do with them?

Problem: control pause, stress, rate and pitch of synthetic speech portably. Conditions: engines that accept SSML. Method: <break/> is an empty element; strength takes none, x-weak, weak, medium (default), strong or x-strong, and time takes a duration (e.g. 250ms or 3s); if neither attribute is given the processor produces a break stronger than it otherwise would; if both are given the time sets the duration and the strength shapes other prosodic changes. <emphasis> requests prominence, but the engine decides how to render it because emphasis differs between languages, dialects and voices. <prosody> controls pitch, contour, range, rate, duration and volume; rate may be a percentage multiplier (100% = unchanged), volume a signed dB value (-6 dB roughly halves amplitude) or a label, and it is an error to use prosody with no attributes. The spec says all prosodic attribute values are indicative (the engine may clamp or substitute) and that authors are encouraged not to casually mix the author's level of control with the processor's own prosody decisions. Use (inference): put pauses at sentence/paragraph structure first, use <break time=...> for exact timing, and treat emphasis/pitch as requests whose effect must be listened to per voice. Failure modes: assuming exact Hz or dB are rendered; splitting a word with <break> (the spec says tokens cannot span markup and the word becomes two tokens). Limitations: the spec defines what to support, not which vendors do; see the vendor findings. Version: W3C Recommendation 7 September 2010. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Spec is from 2010 and vendor engines extend or restrict it; nothing here says what a neural or LLM-based voice does. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://www.w3.org/TR/speech-synthesis11/",
      "note": "3.2.3 break Element; 3.2.3 break Element; 3.2.2 emphasis Element; 3.2.4 prosody Element"
    }
  ]
}