The getElementProperty message handler in testing/marionette/listener.js does not appear to be using the content principal of the element. This means a property set on an element after load as completed will not be accessible. See https://github.com/mozilla/geckodriver/issues/938 for more details. More investigation is needed.
Summary: getElementProperty does not use content principal of element → WebDriver:GetElementProperty does not use content principal of element
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