constructor, what should the url parameter point to?
The url is the “base”, which for a normal URL would be the parent location—the parent directory, or the location where the document you are rendering is located. If your document has absolute URIs for CSS and images, or it has no external references at all, then the base url can be null. If your document has any relative URIs for CSS or images, then the base URL should not be null, but rather should point to the directory or address where the current document is located.
