When display: contents was first implemented by browsers, it was not designed to fully take into consideration how browser engines could implement this in an accessible way. Because it required significant refactor of the entire accessibility system in browser engines, it's taken time to fix... and meant the initial implementations were not at all accessible.

Web developers have been told "don't use this!" Although, of course, not everyone gets the memo, and developers are using it. Especially since subgrid has taken so very long to ship.

Browsers have, over time, greatly improved their implementation of display: contents. It is now behaving properly most of the time / for core use cases. But there are still ways in which it's not yet fixed, and that can create an accessibility minefield.

If there were WPT tests for display: contents, it could be included in Interop 2024. And we could encourage all three engines to finish improving their implementation, squash all the remaining accessibility bugs, and ensure elements with display: contents applied (and all their children) don't magically vanish from screenreaders.