Eric Meyer, the king of css still has his old css references in frames pages https://meyerweb.com/eric/css/references/css1ref.html
> Nightly Can’t Open This Page
> To protect your security, www.w3.org will not allow Nightly to display the page if another site has embedded it. To see this page, you need to open it in a new window.
> Learn more…[1]
> [Open Site in New Window]
[1]: Links to https://support.mozilla.org/en-US/kb/xframe-neterror-page
Dear end-user, do you need therapy? I'll make sure that message is removed in the next release, so other end-users are not subjected to such horribly frightening information.
> Refused to frame 'https://www.w3.org/' because an ancestor violates the following Content Security Policy directive: "frame-ancestors 'self' https://cms.w3.org/".