Reference: https://www.w3.org/TR/speech-synthesis/

This doesn't seem to be a web browser API? CanIUse doesn't track purely server-side standards.

@cvrebert The API is not server-based. SSML parsing simply has not been implemented by browsers, for various reasons.

Relevant browser inquiries, issues, bugs https://lists.w3.org/Archives/Public/www-voice/2017OctDec/0000.html; https://bugs.chromium.org/p/chromium/issues/detail?id=88072; https://bugs.chromium.org/p/chromium/issues/detail?id=795371; https://bugs.chromium.org/p/chromium/issues/detail?id=806592; https://bugzilla.mozilla.org/show_bug.cgi?id=1425523; which collectively appear to indicate a lack of will to implement SSML parsing at the browser by the maintainers ("owners") of Web Speech API at the browsers, particularly where, at least for Chromium/Chrome, the patch kindly provided by speech-dispatcher maintainers https://github.com/brailcom/speechd (see brailcom/speechd#1 (comment); https://bugs.chromium.org/p/chromium/issues/attachmentText?aid=322294) would immediately enable SSML parsing when espeak or espeak-ng is the underlying speech module used by the browser; at Firefox the only reply was "UNTRIAGED".

(FWIW After drawing the conclusion that there was a lack of effort or will to achieve SSML parsing implemention without use of external web services (EUA/fees/etc.) or extensions (https://softwareengineering.stackexchange.com/questions/362911/how-to-extract-ssml-parsing-code-of-espeak-to-implement-ssml-parsing-at-speechsy/) began composing https://github.com/guest271314/SpeechSynthesisSSMLParser for an end-user/client-side/browser JavaScript solution for implementation of SSML parsing following the published standard)