With Intelligent Tracking Prevention, the Safari browser is on a crusade against cross-site tracking. One of the most obvious and long-standing ways to battle cross-site tracking has been to block third-party cookies in the web browser, and this is exactly what Safari does by default. However, Google Tag Manager’s Preview mode relies on a third-party cookies, so that it can serve you the draft version of the container while serving the regular, live container to your site visitors.
Ah, Safari’s Intelligent Tracking Prevention - the gift that keeps on giving. Having almost milked this cow for all it’s worth, I was sure there would be little need to revisit the topic. Maybe, I thought, it would be better to just sit back and watch the world burn. But then Mr. Charles Farina, a good friend and so amicable he’s practically Canadian, tempted me into testing out a quick proof-of-concept web service that would give us back our first party cookies.
Updated 12 March 2019 with some minor clarifications.. On 21st February 2019, WebKit announced the release of the latest iteration of Safari’s Intelligent Tracking Prevention (ITP), known as ITP 2.1. For a while now, Safari has been targeting cross-site tracking with ITP, first starting with cookies in third-party contexts, then tightening the noose after a number of workarounds emerged, and finally with the latest iteration targeting cookies that were moved from a third-party context to a first-party context.
Updated 7 March 2019 - Added some extra caveats to this solution. Also, be sure to read my article on ITP 2.1, which has far more detail on what Intelligent Tracking Prevention is and how to work with it. Looks like Safari is tightening the noose around browser cookies with the introduction of ITP 2.1 (Intelligent Tracking Prevention). Among other things, ITP 2.1 caps the expiration of client-side cookies to 7 days.