= contain-intrinsic-size =

Per [1], 
> These properties allow elements with size containment to specify an explicit intrinsic inner size, causing the box to size as if its in-flow content totals to a width and height matching the specified explicit intrinsic inner size (rather than sizing as if it were empty).

The implementation is roughly split into two parts:
* Get the size from the contain-intrinsic-* properties as an explicit intrinsic inner size.
* Make the sizecontainment box sizing as if its content is the explicit intrinsic inner size.

== The explicit intrinsic inner size ==
  The part has landed in [2].
  * Added `explicitIntrinsicInnerLogicalWidth/Height()` to RenderBox.h, it gets the value of `contain-intrinsic-*` from RenderStyle.
  * To handle the logicalWidth, in `computeIntrinsicLogicalWidths`, the intrinsic size of sizeContainment was zero, now it is explicitIntrinsicInnerLogicalWidth() if existing. We need to handle every `computeIntrinsicLogicalWidths()` in the code.
  * To handle the logicalWidth, in `updateLogicalHeight()`, sizeContainment was treated as if there is no content, now it added explicitIntrinsicInnerLogicalHeight() if existing.
== The size from the contain-intrinsic-* properties ==
  === Value `none` and value `length` ===
    These two values are simple, for `none`, there is nothing to do, for `length`, we need to set the value to RenderStyle.
  === Value `auto length` ===
    The explicit intrinsic inner size is its last remembered size if any, if no, then it is the value of `length`.
    ==== The last remembered size ====
    Per [3], the last remembered size is the size when the element is not size-contained. The value is determined when ResizeObserver events are determined and delivered.
    * The last remembered size is related to Element, we add `RefPtr<ResizeObserverSize> m_lastRememberedSize` to `ElementRareData`. See the implementation in [4].
    * Create a ResizeObserver to handle the last remembered size. See the implementation in [5].
      - To create a specific ResizeObserver, we need to create a callback to handle the last remembered size, it is `CallbackForContainIntrinsicSize`, which removes or saves LastRememberedSize to the target.
      - Document will create and own `m_resizeObserverForContainIntrinsicSize` if there is an element with `contain-intrinsic-size: auto length`.
      - In `RenderTreeUpdater::updateElementRenderer`, it will observe the element if it is with `contain-intrinsic-size: auto length`, if not unobserve it.
      - When `deliverResizeObservations`, the LastRememberedSize will be saved.
  

[1] https://w3c.github.io/csswg-drafts/css-sizing-4/#intrinsic-size-override\\
[2] https://github.com/WebKit/WebKit/pull/1799\\
[3] https://w3c.github.io/csswg-drafts/css-sizing-4/#last-remembered\\
[4] https://github.com/WebKit/WebKit/pull/7375\\
[5] https://github.com/WebKit/WebKit/pull/7829\\