Origin-keyed agent clustering enabled by default

The Origin-Agent-Cluster: HTTP header controls whether a document is isolated in an origin-keyed agent cluster or in a site-keyed agent cluster. This functionality has security implications because an origin-keyed agent cluster allows isolating documents by origin. The consequence of this for developers is that the document.domain accessor can no longer be set when origin-keyed agent clustering is enabled. If you enable or don't configure this policy, documents without the Origin-Agent-Cluster: header are assigned to origin-keyed agent clustering by default. On these documents, the document.domain accessor isn't settable. If you disable this policy, documents without the Origin-Agent-Cluster: header are assigned to site-keyed agent clusters by default. On these documents, the document.domain accessor is settable. For more information, see https://go.microsoft.com/fwlink/?linkid=2191896.