That's a perfectly understandable question!
The short answer is:
Yes, it's possible, but with a significant disadvantage, especially online.
A HUD today is not just an in-game aid; the associated software is an
essential tool for analyzing your own play and that of your opponents away from the tables. Without this data, you lose a crucial part of the necessary studying.
When multi-tabling or on fast-fold tables, a HUD is almost indispensable for quickly spotting opponent tendencies and exploiting them. Even if you're good at taking notes, your brain cannot process as many
hands and statistics simultaneously as software can.
However, on some
poker sites, HUDs are banned, which levels the playing field again. If you focus on these "no-HUD" sites, then you'll be well-equipped with good note-taking and sharp observation skills.
The bottom line: If you want to play seriously online and maximize your win rate, a HUD with tracking software is the
norm—but not an insurmountable obstacle if you're willing to study intensely in other ways and switch to HUD-free sites.