Once you decode UTF-8, iso-latin-1 and cp1252, you end up with Unicode characters, so that doesn't change the problem:

• Determining which Unicode characters can be represented by most browser/computer setups, and
• determining what to do with those that can't.

Yes, you might get undecodable text if you receive something that's in the wrong encoding. And yes, a different character than the intended one might be displayed. But that's an entirely different problem than the one the OP asked about.