Complex sentence: JSON documents may be, and often are, encoded in UTF-8 which supports the full Unicode repertoire; but including characters outside the BMP (U +10000 to U +10FFFF) may be problematic.
Split into simpler sentences: JSON documents may be, and often are, encoded in UTF-8, which supports the full Unicode character set. However, including characters outside the BMP (U +10000 to U +10FFFF) can be problematic.