From what I can think of, its possible to learn the basic fundamentals online that tell you how to read notes, count in 4 4 time, what a melody is, and etc. However, a large part of piano is knowing where to place your fingers so it doesn't tangle up as you progress through the keys upward or downward. It can be confusing at first to the point where you may need someone who has experience with a piano in real life to ensure you are doing it correctly. It is also beneficial to have someone personally help you with sheet music as it can also be confusing if you've never tried to understand one before (assuming you don't know how). Piano requires you to know how to read notes on both the base and treble clef with some degree of dexterity.
While a good portion of this is possible to learn through online (albeit difficult), I think it's a good idea at the very least to borrow someone's piano book. Generally they're more specific and helpful, so you can skim through it and refer back to it if you're ever stuck.
EDIT: Probably a better answer to your question, learning piano can be moderately okay to difficult depending on how much effort you're willing to put in. Learning by yourself is pretty damn difficult, especially if you have little experience with music in general. If you just want to learn the basics, it shouldn't be too brutal, but I'd advise that you get at least some outside help (it doesn't even have to be from a tutor; just a friend who knows about piano) or else you're going to have a rough time.
If you ever want to get into music, then learning the piano is very helpful, so there is a worthy output.