Midrasch zu Wajikra 21:14

1) (Vayikra 21:14) ("A widow, and a divorcée, and a challalah, and a zonah — these he shall not take; but only a virgin from his people shall he take as a wife.") Let it be stated (only) "a widow." Why is "a divorcée (also) written? For if "a widow" (alone) were stated and not "a divorcée," I would say: If a widow, who is permitted to an ordinary Cohein, is forbidden to a high-priest, then a divorcée, who is forbidden to a high-priest, how much more so is she forbidden to a high-priest! If so, I would say: A widow, who is permitted to an ordinary Cohein — his seed from her is profane (vis-à-vis the priesthood); a divorcée, who is forbidden to an ordinary Cohein — his seed from her is a mamzer. It is, therefore, written "a divorcée …" (Vayikra 21:15) And he shall not profane."

2) Or if it were written "divorcée" and not (only) "widow," I would say: A divorcée, who is forbidden to an ordinary Cohein — his seed from her is profane; a widow, who is permitted to an ordinary Cohein — his seed from her is kasher. It is, therefore, written "A widow … And he shall not profane," "A divorcée … And he shall not profane" — "a widow," for severity; "a divorcée," for leniency.

3) "and a challalah": Which is a "challalah"? A woman who was born of one of those forbidden to the priesthood (e.g., the daughter of the union of a widow and a high-priest).