Wikipedia references this part of the Netscape Navigator documentation: https://web.archive.org/web/20020808153106/http://wp.netscape.com:80/eng/mozilla/3.0/han

Wikipedia references this part of the Netscape Navigator documentation: https://web.archive.org/web/20020808153106/http://wp.netscape.com:80/eng/mozilla/3.0/handbook/javascript/advtopic.htm#1009533 https://web.archive.org/web/20020808153106/http://wp.netscap... This suggests that the same-origin policy was introduced rather quickly after the introduction of Javascript, as a security fix.