Is the order of union all guaranteed?

When using UNION ALL in MySQL, if there is no ORDER BY, the elements will basically return in the order of UNION.

Question

• Can I expect the UNION ALL set to return elements in the order of UNION as MySQL or SQL specifications?

• Or, if certain conditions (storage engine, etc.) are met, will each element of UNION ALL be acquired in a mix?

As a MySQL or SQL specification, can I expect UNION ALL sets to return elements in the order of UNION?

Or, if certain conditions (storage engine, etc.) are met, will each element of UNION ALL be acquired in a mix?