Happy (played by Iron Man director/producer Jon Favreau) was Tony Stark's valet until Iron Man 3, in which he gets seriously injured in an attack by the Mandarin. Thankfully, by the end of the movie, he's recuperating, even enjoying some Downton Abbey in the hospital. And that's when some comic book / British TV fans went "Oh COME THE f**k ON! Hasn't he suffered enough?!"
Downton Abbey is a historical drama about the rigors of Britain's class-based society, best exemplified by the relationship between the aristocratic Lady Sybil and Tom Branson, who initially works as her family's chauffeur. They eventually fall in love and get married, defying the norms of their time and resulting in the Great Monocle Drop of 1919. And Happy is watching a scene between those two characters from his hospital bed.
He even gets really emotional over it. That's because, like Tom, Happy is a driver for a rich guy and in love with someone way out of his league: Tony's assistant turned executive turned girlfriend Pepper Potts.
There were tiny signs of Happy's true feelings in the previous movies, but this scene confirmed it to geeks in the know. In the original comics, Happy and Pepper were married. They fell in love after Happy was almost killed by one of Iron Man's enemies -- like what happens in Iron Man 3, except without the upside. So the Downton Abbey clip is a clever nod to how Happy feels about Pepper, a kind of reward for fans of the comics. Or a kind of punishment, because the MCU Pepper and Happy never get together. While she's with Tony, Happy gets to watch and cry into his whiskey every night. Still think reading is cool?