Yikes. Chrome-style Site Isolation often can’t protect against cross-origin cookie disclosure unless they’re SameSite.https://twitter.com/wycats/status/948812652997885952 …
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I believe the web developer advice is orthogonal to the user advice there. Devs should be mitigating without assuming any kind of isolation by the user agents.
The developer advice is completely untenable, so if it's intended to be needed long term, we have some work to do.
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