[css-sizing] Define precise timing for setting last remembered size?

From https://drafts.csswg.org/css-sizing-4/#last-remembered, the last remembered size is recorded

At the time that ResizeObserver events are determined and delivered

But what if the author is already observing the element with some ResizeObservers? When an observed element changes size, the various callbacks are invoked in order of the creation of the ResizeObservers. And the callbacks can check whether the last remembered size has been recorded or not.

Then webpages can tell that both Firefox and Chromium:

• Reuse the same ResizeObserver instance for updating the last remembered size of all elements that need it.
• This ResizeObserver instance is created the 1st time that an element with contain-intrinsic-size: auto and no size containment is laid out.
• This ResizeObserver instance is never removed (during the lifetime of the document).

Are we fine exposing these implementations details? Should we define a more precise timing, e.g. that the last remembered size is set just before invoking the (other) ResizeObservers callbacks for that element?

Read the CSS Sizing Level 4 section on last remembered size and the linked Resize Observer specification. Compare the stated timing with the reported Firefox and Chromium behavior, including observer callback ordering and the lifetime of the shared ResizeObserver. Done means resolving whether implementation details should remain observable or defining more precise timing.