2026-07-27 UTC
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tdrayson It was added by Jetpack to enhance the WordPress.com Reader. From what I read in the github thread, although technically it complies with the XML and Namespace spec (https://github.com/Automattic/jetpack/issues/5219#issuecomment-250357987) Since it's technically "unknown" some readers just bug out and don't know how to handle it, rather than just ignoring it.