1968: ASCII
[Overview]
Used since the beginning of computing, ASCII (American Standard Code for Information Interchange) is a 7-bit coded character set for information interchange in English. It was published in 1968 by ANSI (American National Standards Institute), with an update in 1977 and 1986. The 7-bit plain ASCII, also called Plain Vanilla ASCII, is a set of 128 characters with 95 printable unaccented characters (A-Z, a-z, numbers, punctuation and basic symbols), i.e. the ones that are available on the English/American keyboard. Plain Vanilla ASCII can be read, written, copied and printed by any simple text editor or word processor. It is the only format compatible with 99% of all hardware and software. It can be used as it is or to create versions in many other formats. Extensions of ASCII (also called ISO-8859 or ISO-Latin) are sets of 256 characters that include accented characters as found in French, Spanish and German, for example ISO 8859-1 (Latin-1) for French.
[In Depth (published in 2005)]
Whether digitized years ago or now, all Project Gutenberg books are created in 7-bit plain ASCII, called Plain Vanilla ASCII. When 8-bit ASCII (also called ISO-8859 or ISO-Latin) is used for books with accented characters like French or German, Project Gutenberg also produces a 7-bit ASCII version with the accents stripped. (This doesn't apply for languages that are not "convertible" in ASCII, like Chinese, encoded in Big-5.)
Project Gutenberg sees Plain Vanilla ASCII as the best format by far. It is "the lowest common denominator." It can be read, written, copied and printed by any simple text editor or word processor on any electronic device. It is the only format compatible with 99% of hardware and software. It can be used as it is or to create versions in many other formats. It will still be used while other formats will be obsolete (or are already obsolete, like formats of a few short-lived reading devices launched since 1999). It is the assurance collections will never be obsolete, and will survive future technological changes. The goal is to preserve the texts not only over decades but over centuries. There is no other standard as widely used as ASCII right now, even Unicode, a universal double-byte character encoding launched in 1991 to support any language and any platform.