It's plain enough from the Laws of Hywel that slavery existed in medieval Wales. It would be strange if it hadn't since it was an institution that was found in nearly every human society, from the Maoris of New Zealand to the Aleuts of Alaska.
For most of us slavery means the chattel slavery found in North America and the Caribbean - a somewhat Eurocentric outlook on such a universal and continuing phenomenon - and as S4C have gone to some trouble to point out the Welsh played a part in all of this. How else could John Henricus, for example, a runaway slave from New York in 1727, be described as speaking very good English and the Welsh dialect. Incidentally runaway bond servants were just as numerous as runaway slaves and pursued with equal vigour, they sometimes ran away together.
A rare exception to those who saw slavery as just a normal part of life was a Pennsylvania Quaker named Cadwalader Morgan, who, in 1696, after much pondering over the practicalities of owning a slave, decided that he had "no freedom to buy or take any of them upon any account." He took his message to the Quaker's Philadelphia Yearly Meeting, which, although it rejected the call to forbid slavery, did agree that Friends "be careful not to encourage the bringing in of any more Negroes."
Cadwalader Morgan had emigrated to Pennsylvania from Merionethshire but, as Charles Browning's Welsh Settlement of Pennsylvania points out, his will of 1711 shows him to have been the son of James Morgan from the township of Faenor in Nantmel parish. Cadwalader, who had married into a Merionethshire Quaker family, migrated to Pennsylvania in 1683. His parents, three brothers and a sister sailed out to America in 1691; both father and mother died on the voyage.
It's interesting that Morgan based his opposition to slavery on practicalities rather than principle - he felt that owning a slave could have a negative moral impact on the owner and his household. The abolitionists of 19C America also had to face practical concerns; how exactly could one emancipate what, in some states, amounted to 40% of the population without causing economic and social chaos. In the end the matter was decided on the battlefield, with one soldier dying for every six slaves freed.