The contain-intrinsic-size property allows developers to specify a placeholder size which would be used while contain: size is applied. With contain-intrinsic-size specified, elements lay out as if they had a single child with fixed size (specified by contain-intrinsic-size) unless they have an explicit width/height. This is helpful when size containment may be dynamically added/removed so that there is a fallback size during size containment but a content-based size during regular sizing.
The motivation for the property is to provide a placeholder sizing for subtree content which is either not yet available or not rendered. There is currently no way to provide this, other than sizing the element itself which may not be desirable as it affects how the element lays out in its container.
Docs: https://github.com/w3c/csswg-drafts/issues/4229
Explainers: https://github.com/WICG/display-locking/blob/master/explainer-contain-intrinsic-size.md