By choosing a conservative suit to wear to Capitol Hill, Facebook CEO Mark Zuckerberg signaled his seriousness about the occasion

Facebook followers were eager to see what CEO Mark Zuckerberg chose to wear to testify before Congress Tuesday —a clue into how he perceived the event and how he wanted to be perceived. When building his company, the 33-year-old billionaire famously favored T-shirts, hoodies, jeans and sneakers—a schlubby-collegiate style that infuriated some who saw him flouting traditional business wear.

However, over the years the Facebook chief has shown he can dress up when the occasion—and the institution—calls for it, such as a visit to the Vatican, the White House and now Capitol Hill. That’s a big step for a man once loath to wear a suit, much less a tie. And when Mr. Zuckerberg does dress up these days, the suits aren’t wearing him. Even his casual ensembles have been looking more put-together lately. “He still wears hoodies and T-shirts but I think they fit more properly,” said Hollywood stylist Jenny Ricker, whose clients include Zac Efron, Joseph Gordon-Levitt, and Kit Harington. Plus, “it’s not like Hanes, but probably like [contemporary fashion brands] John Elliott or Rag & Bone, something you’d buy at Barneys.” Ms. Ricker adds that from Instagram images of Mr. Zuckerberg, she’d guess “he might tailor his T-shirts. His arms fit perfectly.”