The [[ https://www.w3.org/TR/css-speech-1/#speaking-props-speak | CSS Speech Module Level 1 ]] (CR draft, 14 February 2023) `speak` property is now flagged as "Unrecognized or unsupported property at line //x// character //y//." by css-sanitizer when attempting to save a sanitized-css page containing it.
The case at hand is [[ https://en.wikisource.org/w/index.php?title=Template:Header/styles.css&oldid=11511437 | s:Template:Header/styles.css ]]. If I just edit that page and immediately save it I get the error message, because the `speak` property apparently worked on [[ https://en.wikisource.org/w/index.php?title=Template:Header/styles.css&diff=prev&oldid=10859211 | 22 January 2021 ]] when it was first added to the page.
On pages with CSS content model (like [[Special:MyPage/common.css]]) it works fine. But any TemplateStyles page (i.e. sanitized-css) I tested I got the same behaviour.
Now admittedly my codesearch-fu isn't all that, but as best I can figure out this is due to rCSSSebf03becffd5 by @Anomie on 6 February 2022. According to the commit message that commit drops CSS Speech entirely because it is "obsolete", which I think must be a mistake. The old (2012) [[ https://www.w3.org/TR/2018/NOTE-css3-speech-20180605/ | CSS Level 3 Speech Module]] was retired in 2018, but that was just the normal diffusion of CSS specs to replace it with [[ https://www.w3.org/TR/2020/CR-css-speech-1-20200310/ | CSS Speech Module 1]] (which was current from 10 March 2020). At time of writing the 14 February 2023 CR draft is current, and just skimming the two versions suggests this is indeed just a revision rather than a completely new spec.