[css-speech-1] CSS Speech 1 should not move to Rec until there are at least 2 implementation combinations of 2 combinations of "user agent + platform API + assistive technology" for every feature

Perhaps this requirement should go without saying, but I'm listing it here as a blocking issue for css-speech-1 because:

• CSS 3 Speech went to PR/TR without implementations.
• Since that time (6+ years?), there have been few implementations of a few features, and no implementations of many features.
• As far as I know, most css-speech-1 features remain unimplementable by the user agents on most platforms because there is a lack of Accessibility API support. That could change, but it would need an incubation effort.

Unlike most CSS features, Speech is rarely (if ever) rendered by the user agent. As an example, fallback fonts are handled by the UA rendering engine so it can implement, or not, without dependency. Speech in most (perhaps all?) mainstream assistive technology is not rendered by the UA. Speech generated from web pages requires the rendering engine to expose certain properties or features through the platform accessibility API. The API in turn is accessed by the assistive technology, which renders the speech.

I’m not aware of API support for fallback voice electives (and many other CSS Speech features) in accessibility APIs so there is a gap here. Some AT supports language fallbacks because the lang attribute is exposed (e.g. as AXLanguage in the Mac AX API), but in this case, the AT can’t render what is not exposed, and the UA has no way to expose it. I believe the same is true on other platform APIs.

Again, this could change, but it would need an incubation effort to convince a combination of at least 2 platform API maintainers and 2 assistive technology developers that this is a priority. The "2 implementations" requirement applies to 2 combinations of "user agent + platform API + assistive technology." ARIA went through a similar testing requirement.