Make 'Cell<JS<T>>' traceable.
Make 'RefCell<Vec<JS<T>>>' traceable.
Use Cell/RefCell for interior mutability of Attr, AttrList, Blob,
BrowserContext, ClientRect, and ClientRectList.
Make 'Cell<Option<JS<T>>>' traceable.
Use Cell/RefCell for interior mutability of Document.
Use Cell/RefCell for interior mutability of DOMImplementation and DOMParser.
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>>.