Description

While all the browsers have shipped the basic functionality - there have been a number of improvements agreed to by the CSS Working Group, which have not been implemented consistently. The first two are especially high priority, since they have compatibility implications:

• Queries resolve against the flat tree: [css-contain] CQ vs shadow boundaries w3c/csswg-drafts#5984 (comment)
• Queries with unknown features, and comma-separated syntax: [css-contain-3] Inconsistent handling of known and unknown features jeopardizes backward compatibility w3c/csswg-drafts#7551 (comment)
• Container font units: [css-contain-3] Add container-font relative units  w3c/csswg-drafts#8855
• Explicit container names in units: [css-contain-3] Reference named containers for cq units w3c/csswg-drafts#7858
• Custom properties in dimensional query values: [css-contain-3] Can we allow custom properties in dimensional container queries? w3c/csswg-drafts#8088
• Optional <container-query> prelude: [css-contain-3] Make <container-query> optional in @container w3c/csswg-drafts#9192

Specification

https://drafts.csswg.org/css-conditional-5/

Additional Signals

WPT: https://wpt.fyi/results/css/css-conditional/container-queries

Chromium status or issue:

• Queries resolve against the flat tree: https://chromestatus.com/feature/5242724333387776
• Queries with unknown features: https://chromestatus.com/feature/5105423604776960
• Container font units: https://crbug.com/40277671
• Explicit container names in units: https://crbug.com/40261733
• Custom properties in dimensional query values: none
• Optional <container-query> prelude: https://crbug.com/40287550

I haven't had a chance to track down other status/issues. I'll try and get to that, or would be happy for help.