Take the 2-minute tour ×  I am desiging a file format and I want
to do it right. Since it is a binary format, the very first byte
(or bytes) of the file should not form valid textual characters
(just like in the PNG file header1). This allows tools that do
not recognize the format to still see that its not a text file
by looking at the first few bytes.  Any codepoint above 0x7F is
invalid US-ASCII, so that's easy. But for Unicode it's a whole
different story. Apart from valid Unicode characters there are
private-use characters, noncharacters and sentinels, as I found
in the Unicode Private-Use Characters, Noncharacters & Sentinels
FAQ.  What would be a sentinel sequence of bytes that I can use
at the start of the file that would result in invalid US-ASCII,
UTF-8, UTF-16LE and UTF-16BE?    • Obviously the first byte
cannot have a value below 0x80 as that would be a valid US-ASCII
(control)character, so 0x00 cannot be used.   • Also, since
private-use characters are valid Unicode characters, I can't use
those c