Explainer in the context of display locking (a motivating example): https://github.com/WICG/display-locking/blob/master/explainer-new.md
Explainer in the spec draft (introduction section):
The content-size size property allows the developer to specify a placeholder size which would be used for content under size containment which is not yet rendered or ready for display. Currently, with size containment, elements lay out as if they were empty. With size containment and content-size specified, elements lay out as if they had a single child with fixed size (specified by content-size).
Interoperability and Compatibility
This is a new feature which can be feature detected. If unavailable, the behavior may be approximated by sizing the size-contained element directly. In other words, the desired behavior can be reproduced on a case-by-case basis with manipulating the element size.
Other UAs: No signals yet
Web developers: No signals yet
The content-size property only has an effect when size containment is present, and extends size containment by allowing content to be sized by the developer.
Link to entry on the feature dashboard
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