I'm perplexed by the same thing. I was working on my pop ups in AGOL, but the window there is fairly small for using custom HTML formatting.
I was hoping to use Pro for a better interface and found that it didn't even offer the basic text formatting options that AGOL does:
AGOL
PRO (no font size options??)--I don't see HTML options anywhere, though I recall ArcMap having this capability.
I feel like I'm playing ping pong with myself between the two products!!