imported/w3c/web-platform-tests/html/browsers/browsing-the-web/history-traversal/persisted-user-state-restoration/scroll-restoration-fragment-scrolling-cross-origin.html does the following: iframe.src = iframe.src.replace("http://", "http://www.").replace("page-with-fragment.html#fragment", "blank1.html"); which sets the frame source to http://www.localhost:8800/html/browsers/browsing-the-web/history-traversal/persisted-user-state-restoration/resources/blank1.html We seem to block this: Blocked access to external URL http://www.localhost:8800/html/browsers/browsing-the-web/history-traversal/persisted-user-state-restoration/resources/blank1.html Also loading that url in Safari fails (when the server is running), but chrome loads it fine. I think maybe we have to treat subdomains of localhost specially?