ReadonlyblockThe blockSize read-only property of the ResizeObserverSize interface returns the length of the observed element's border box in the block dimension.
ReadonlyinlineThe inlineSize read-only property of the ResizeObserverSize interface returns the length of the observed element's border box in the inline dimension.
The
ResizeObserverSizeinterface of the Resize Observer API is used by the ResizeObserverEntry interface to access the box sizing properties of the element being observed.MDN Reference