Throughout its history, television has done period drama with varying degrees of success. Mad Men 's production design seems meticulously researched compared to period pieces from American television's earlier periods. How do Gunsmoke or Bonanza hold up next to Deadwood (or even '90s syndie Lonesome Dove: The Outlaw Years)? From its second season, Happy Days included more anachronisms (costumes, grooming, dialog) in each episode than we will probably see in Mad Men's entire run.
Productions take more care with historical material in the era of Deadwood and HBO's various biographical films, but there are still exceptions. We'll cut the recent BBC production of Robin Hood some slack for its obvious camp but that still doesn't excuse its lapses into modern dialog ("Okay!") and off-the-rack costumes. The Tudors is Showtime's wretched retelling of the Henry VIII history in which only a few of the supporting roles are even watchable. Despite the richness (and middling accuracy) of the interiors and the richness (and irritating inaccuracy) of much of the costuming and grooming, the series' grating dialog and petulant characterizations are enough to make the discerning viewer long for the late '60s and early '70s run of BBC and ITV studio productions that set a seldom-matched standard for televised historical drama.
Some of those British series were, like Mad Men, original stories and not adaptations, among them The Six Wives of Henry VIII and Upstairs, Downstairs. They were quite simply very well-written and period-accurate plays.
Straight out of the theatrical tradition, these were dramas performed live-to-tape under the constraints of what is now considered primitive studio TV production: they were shot by relatively huge and cumbersome television - not video - cameras, under harsh lights on a few standing sets. Yet the beauty of the dialog, the quality of the performances, and the rich detail of the costumes (copied, albeit cheaply, from portraits and other primary sources) combine to pull the viewer into another place and time, and keep us there, which is exactly what we want from a period piece.