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The US Library of Congress has some pragmatic notes on these formats. In practice, therefore, the Strict format is less well supported than Transitional – even though Microsoft has offered Strict as an option since 2012.
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This is the issue referenced by The Document Foundation above yet LibreOffice itself saves in the Transitional format, and an open issue to support Strict gets little attention. A further complication is that in theory a variety of OOXML called Strict is the preferred standard, but in practice everyone (including Microsoft) uses OOXML Transitional, and realistically it seems that the period of transition may never end.
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