Currently we have support for:

• Cover
• Image
• Featured Image
• Group
• Columns (and Column)

We need to add support for more blocks to ensure an optimal sizes attribute, especially Gallery (see #1511 (comment)).

Maybe we need to support the Grid block too, and the Row block would be very similar to the Columns block.

The functionality here provides huge performance gains in my testing, so the better coverage we have for all the core blocks the better. And once that support is present, I think we should proceed with core merge proposal.

See also:

• Incorporate layout constraints from ancestor blocks into the sizes calculations #1511