Use Cell/RefCell for interior mutability of Element.
Implement OptionalSettable for 'Cell<Option<JS<U>>>'.
Use Cell/RefCell for interior mutability of Event.
Use Cell/RefCell for interior mutability of FormData, BlobData.
Use Cell/RefCell for interior mutability of HTMLCollection.
Use Cell/RefCell for interior mutability of MouseEvent.
Use Cell/RefCell for interior mutability of NodeList, Performance, TestBinding, and ValidityState.
Use Cell/RefCell for interior mutability of UIEvent.
Use Cell/RefCell for interior mutability of XMLHttpRequest.
Use Cell/RefCell for interior mutability of Window.
 Use Cell/RefCell for interior mutability of Frame.
Use Cell/RefCell for interior mutability of Node.
Remove the imple OptionalSettable for Option<JS<T>>.