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.

Elon Musk and Donald Trump can’t seem to stay away from each other. Are they just work buddies, or are they best friends? The tech billionaire and the president have spent a lot of time together this year,