Earlier quoted context omitted.
Firefox on Android actually has a great error message here: > 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
Oh my god! You must be very confused now, being told an error message, and some detail about it. 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/".