What does W3C SSML 1.1 say the emphasis element must do, and which levels exist?

Problem. Whether <emphasis> gives portable, predictable stress. Conditions. Any SSML 1.1 processor. Method. Wrap the word or phrase in <emphasis level=...>; levels are strong, moderate (default), none and reduced; none, moderate and strong are monotonically non-decreasing. 'reduced' is the opposite of emphasis (the spec's example is 'going to' becoming 'gonna'); 'none' stops a processor from stressing a word it would normally stress. The element may contain text and audio, break, emphasis, lang, lookup, mark, phoneme, prosody, say-as, sub, token, voice and w. Failure modes. The spec leaves rendering to the processor, so the same markup differs across engines and voices. Limitations. No acoustic values (Hz, dB, ms) are defined. Version and date. SSML 1.1, W3C Recommendation, 7 Sept 2010 (section 3.2.2). Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Rec is from 2010; engines may ignore or partly implement it. Spec text read in full for this section only. 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": "cited at https://www.w3.org/TR/speech-synthesis11/; 3.2.2 emphasis Element; 3.2.2 level attribute; 3.2.2 level attribute"
    },
    {
      "url": "https://www.w3.org/TR/speech-synthesis11/",
      "note": "observed URL (dated/alias form of the cited catalogue source; canonical identity retained)"
    }
  ]
}