WPT details: https://wpt.fyi/results/css/css-anchor-position?label=master&label=experimental&aligned&q=position-visibility
The `anchors-valid` value was implemented as part of https://crbug.com/329703412 but subsequently reverted, due to spec questions about the exact definition of the value. Since that time, the value has been renamed to `anchor-valid` (see https://chromestatus.com/feature/5136123851571200) and also better defined to be clear that only the default anchor matters, resolving a lot of ambiguity, and making this value shippable.