What is the status of a marriage witnessed by an SSPX priest?

What would be the status of a marriage between baptized Catholics that was witnessed by a priest of the Society of St. Pius X (SSPX)?

Such a marriage would be presumed invalid because priests of the Society of St. Pius X are not delegated by Catholic bishops to witness the marriages of Catholics.

Recommended reading:

Marriage Involvement 4 (SSPX) by Jimmy Akin
SSPX Confessions by Jimmy Akin

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