spec draft: 

```
browsingContext.createContainer(Extensible) => id
browsingContext.closeContainer(id) # without running before unload
browsingContext.getContainers() # only IDs are needed

browsingContext.CreateParameters = {
  type: browsingContext.CreateType,
  ? referenceContext: browsingContext.BrowsingContext,
  ? background: bool .default false
  ? container: BrowsingContext.Container / null
}

browsingContext.GetTreeParameters = {
  ? maxDepth: js-uint,
  ? root: browsingContext.BrowsingContext,
  ? container: BrowsingContext.Container / null
}

browsingContext.Info = {
  context: browsingContext.BrowsingContext,
  url: text,
  children: browsingContext.InfoList / null
  ? parent: browsingContext.BrowsingContext / null,
  ? container: BrowsingContext.Container / null
}
```