Iframe credentialless give developers a way to load documents in third party iframes using new and ephemeral contexts. Iframe credentialless are a generalization of COEP credentialless to support 3rd party iframes that may not deploy COEP. Like with COEP credentialless, we replace the opt-in of cross-origin subresources by avoiding to load non-public resources. This will remove the constraint that 3rd party iframes must support COEP in order to be embedded in a COEP page and will unblock developers looking to adopt cross-origin-isolation. This way, developers using COEP can now embed third party iframes that do not.
Sites that wish to continue using SharedArrayBuffer must opt-into cross-origin isolation. Among other things, cross-origin isolation will block the use of cross-origin resources and documents unless those resources opt-into inclusion via either CORS or CORP. This behavior shipped in Chrome, Firefox and Safari. The opt-in requirement is generally positive, as it ensures that developers have the opportunity to adequately evaluate the rewards of being included cross-site against the risks of potential data leakage via those environments. It poses adoption challenges, however, as it does require developers to adjust their servers to send an explicit opt-in. This is challenging in cases where there's not a single developer involved, but many. Google Ads, for example, includes third-party content, and it seems somewhat unlikely that they'll be able to ensure that all the ads creators will do the work to opt-into being loadable. It seems clear that adoption of any opt-in mechanism is going to be limited. From a deployment perspective (especially with an eye towards changing default behaviors), it would be ideal if we could find an approach that provided robust-enough protection against accidental cross-process leakage without requiring an explicit opt-in.
Docs: https://docs.google.com/document/d/1poI75BaQ9aqcMGJn_K01-QHsQQbEOwRWvg3Af4VWTmY/edit
Samples: https://anonymous-iframe.glitch.me
Explainers: https://github.com/WICG/anonymous-iframe