[...] And the ISO C standard paper I found says:
The external representations in a text file need not be identical to the internal representations, and are outside the scope of this International Standard.
NOTHING ELSE! Aside that lines must at least be 254 bytes long:
An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.
The official ISO C standard says that too (ISO/IEC 9899:1999 (E), page 263):
Data read in from a text stream will necessarily compare equal to
the data that were earlier written out to that stream only if: the
data consist only of printing characters and the control characters
horizontal tab and new-line; no new-line character is immediately
preceded by space characters; and the last character is a new-line
character.