After reading Case C-40/17 and Case C-210/16 I still cannot figure out why GDPR would require "joint controller" decision.
Wouldn't it be much simpler to decide that e.g. in "Fashion ID" case, the Facebook cannot set cookies if Facebook hasn't acquired consent from the end user? That would allow embedding Like buttons on any page at will and Facebook could only track the users that have already given consent for Facebook. Technically facebook would have been able to run their own JavaScript code on the host page so they could have implemented the consent query on any page if they really wanted to set cookies only after getting consent.
This would also immediately solve questions such as Who's liable for GDPR compliance when embedding/hosting a self-contained 3rd-party website/service? and Who needs to implement Cookie Consent for embedded content? because in both cases the host that wants to set non-necessary cookies would need to independantly verify they already have been given consent for those cookies. As GDPR doesn't define how such content must be acquired, there could be an UI on the host domain document and then the acquired consent must be explicitly transferred to 3rd party. In practice, the transfer cannot be just "it's okay" but "user has given consent for trackingdomain.com via domain example.com at yyyy-mm-dd with log id abc123ef" because GDPR requires that the user must be able to retract the consent later and entity doing the processing must be able to specify the origin for the consent.
Is the current ruling just caused by the interpretation that embedding any content via URL is considered implicit consent for the whole domain?
In practice, this would practically prevent 3rd party cookies for any services the user hasn't expicitly visited previously.