Mr. Musk is fundamentally a creature of Silicon Valley, a scene that initiated an entire revolution in power dressing. The young masters of that universe dressed down to show that they were members of a new elite — one that refused to dress like the grown-ups they were about to topple. It was the defeat of the suit by the T-shirt.

President Trump’s first Cabinet meeting will include one notable administration figure who is not in the Cabinet. The White House press secretary, Karoline Leavitt, said during a press briefing on Tuesday that Elon Musk would attend the Cabinet meeting.