#ezio.melotti: """I'm considering valid all the bytes that start with '10...'"""

Sorry, WRONG. Read what I wrote: """Further, some bytes in the range 80-BF are NOT always valid as the first continuation byte, it depends on what starter byte they follow."""

Consider these sequences: (1) E0 80 80 (2) E0 9F 80. Both are invalid sequences (over-long). Specifically the first continuation byte may not be in 80-9F. Those bytes start with '10...' but they are invalid after an E0 starter byte.

Please read "Table 3-7. Well-Formed UTF-8 Byte Sequences" and surrounding text in Unicode 5.2.0 chapter 3 (bearing in mind that CPython (for good reasons) doesn't implement the surrogates restriction, so that the special case for starter byte ED is not used in CPython). Note the other 3 special cases for the first continuation byte.