What is "validly-ordained" and why can't I attend any such priest's Masses?

From what I understand SSPX priests and Orthodox priests are “validly-ordained”. If this is so, why is it wrong to attend Mass at these types of churches? What constitutes being “validly-ordained” a priest, if the priest is not in full communion with the Holy See?

Once ordained a bishop, one never loses the power to validly ordain others. One may lose the right to ordain and, therefore, ordain illicitly, but he still retains the power to ordain validly.

If your diocesan bishop and some of his priests separated from the Catholic Church would you think it OK to participate in their Masses? Of course the answer is no as you would be supporting their schism and risking your own separation from the Church. Similarly, it is generally not OK to attend an SSPX Mass or to participate at an Orthodox service in lieu of a Catholic Mass. (Note: There are times when a Catholic may receive sacraments from SSPX or Orthodox ministers - see Code of Canon Law 844 §2.)

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