What does the spec have to say about the expected behavior, does it say anything about preventing xorigin iframes from establishing appcaches?

Removing that function could easily break existing uses.

A reasonable constraint on the number of master entries might be a safer way to go.

Firefox's behavior matches chrome's, it caches the master entries too.

I reckon Safari is (wilfully) breaking spec here, but it does prevent the attack.

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I think google drive and google docs rely on being able to establish appcaches xorigin to pull down new docs.

The safest change i can think of is to put a reasonable limit on the number of master entries, when n+1 comes around, silently refuse to add it. We could add uma stats to get an idea of how many master entries there typically are as the basis for the reasonable limit?

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UMA metric to see if we can remove this?