title: How do YouTube and Gmail share login session when third-party cookies are blocked
body: <p>With Safari 14 now blocking all third-party cookies by default, it surprised me to see that youtube.com automatically logs you in with the same account as with you are currently logged in with at gmail.com. The same session sharing is probably going on between other Google products.</p>
  <p>I am curious about the technical details about how this is possible: How is YouTube able to detect my login session from Gmail, without using third-party cookies?</p>