Oh, I rembered why I added the argument. The documentation mentions that an instance of `Document` is
> Typically obtained from `HTML.render()`, but can also be instantiated directly with a list of `pages` and a set of `DocumentMetadata`.

So there might have been cases where no `url_fetcher` is available.

Do you want me to add an `url_fetcher` attribute to the `Document` class instead (passed to `__init__`)? Is the assumption valid that a `url_fetcher` is mandatory?