The reason is always the same: the content and the key that decrypts it are both present on the client’s machine. The user’s hardware decrypts the content to display it. The user’s hardware is, definitionally, something the user controls. Any sufficiently motivated person with the right tools can intercept the decrypted output.
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Платон Щукин (Редактор отдела «Экономика»)
An creates a completely separate browsing context with its own window object, its own document, and most importantly;its own prototype chain. A function hooked on HTMLMediaElement.prototype in the parent window is not the same object as HTMLMediaElement.prototype in the iframe’s window. They’re entirely separate objects. If fermaw’s audio element lived inside an iframe, my prototype hook in the parent window would never fire.