Right now, section 4.4.1.1. fragment URLs say that:

If a url()’s value starts with a U+0023 NUMBER SIGN (#) character, parse it as per normal for URLs, but additionally set the local url flag of the url().

When matching a url() with the local url flag set, ignore everything but the URL’s fragment, and resolve that fragment against the current document that relative URLs are resolved against. This reference must always be treated as same-document (rather than cross-document).

We text should be updated to refer to the current tree so that when the local url flag is set, the fragment is resolved against the current tree.