Donations inter vivos, mortis causa subject to estate taxes?

Under Section 91 of the Tax Code, donations inter vivos are subject to donor's taxes while donations mortis cause are subject to estate taxes under Section 77 of the same Code. However, even if the donation appears to be inter vivos if the transfer is contemplated to take effect upon death of the donor or is revocable during his lifetime, it may still be taxed for estate taxation purposes.