Jamieson, Fausset and Brown on Deuteronomy 14:21

21. Ye shall not eat of any thing that dieth of itself--(See on Le 17:15; Le 22:8).

thou shalt give it unto the stranger that is in thy gates--not a proselyte, for he, as well as an Israelite, was subject to this law; but a heathen traveller or sojourner.

Thou shalt not seethe a kid in his mother's milk--This is the third place in which the prohibition is repeated [Ex 23:19; 34:26]. It was pointed against an annual pagan ceremony (see on Ex 23:19; Ex 34:26).

[De 14:22-29. Law of the Tithe].