How exactly does site-isolation prevent cross-origin communication that doesn't rely on SharedArrayBuffer, i.e. that vast majority of use-cases? It's just message passing.

I can see that site-isolation is arguably too expensive on mobile and why you might want an opt-in mechanism there, somewhere down the line.

However, I don't think there are good arguments for not just enabling it on Desktop right now, without making developers jump through hoops. Until Chrome enables SharedArrayBuffers on mobile, I have no reason to care anyway.

It doesn’t need to, since that communication is consensual: the sender must explicitly send the information, and the receiver must explicitly be interested in it (and can check what origin it is from). The problem with SharedArrayBuffer (with Spectre) is that is allows the “receiver” to read whatever it wants from the other origin, just by virtue of ending up in the same browser context.

Site isolation disables all of it. With "Cross-Origin-Embedder-Policy: require-corp," you can't even embed a cross-site image unless the other image allows it with a "Cross-Origin-Resource-Policy: cross-origin"

Enabling that on desktop today would break every website that embeds cross-origin images, e.g. everybody using a separate CDN for images would be broken.

You're describing how this proposed cross-origin isolation scheme works. I understand that, I don't understand why it is necessary to make it work that way.

Chrome has been doing site isolation with multiple processes for a for a while, it "just works" and it doesn't break sites.

Site isolation and origin isolation are separate concerns. In the "origin isolation" model, you need to ensure different origins are in different processes, and that their data don't leak from one to the other. In site isolation, you only care about tabs not being able to communicate with each-other.

Also, you seem to be missing something: Chrome is going to implement the same set of headers, with the same set of restrictions when they are applied. This isn't an arbitrary firefox decision, every web browser is expected to follow suit. See the various mentions of "chrome" in https://web.dev/coop-coep/

> In site isolation, you only care about tabs not being able to communicate with each-other.

That is not true.

https://www.chromium.org/Home/chromium-security/site-isolati...

Chrome’s site isolation doesn’t solve the “image from another origin” problem. Those still exist in the containing origin process’s memory. It solves the “frame from another origin” problem, which is the more acute issue but not the only one.