[resize-observer] Why are inlineSize and blockSize unrestricted?

[Loirooriol]

https://drafts.csswg.org/resize-observer-1/Overview.html#resizeobserversize

interface ResizeObserverSize {
    readonly attribute unrestricted double inlineSize;
    readonly attribute unrestricted double blockSize;
};

Why make them unrestricted? They are readonly, so they can't be set from a script. They are only set in calculate box size, and I don't think that can be NaN or infinity.

There is no other CSS spec using a readonly unrestricted attribute.