> The ASCII standard has nothing to say about backspace overstriking
It seems impossible to get ANSI copies of old standards (even for money) but the ECMA (1973) printing says:
3.2 Diacritical Signs
(Positions: 2/2, 2/7, 2/12, 5/14, 6/0, 7/14)
In the 7-bit character set, some printing symbols may be
designed to permit their use for the composition of acce‐
nted letters when necessary for general interchange of
information. A sequence of three characters, comprising
a letter, BACKSPACE and one of these symbols, is needed
for this composition; the symbol is then regarded as a diacrit‐
ical sign. It should be noted that these symbols take
on their diacritical significance only when they precede or
follow the character BACKSPACE; for example, the symbol
corresponding to the code combination 2/7 normally has the
significance of APOSTROPHE, but becomes the diacritical
sign ACUTE ACCENT when preceded or followed by the character
BACKSPACE.
This is precisely the reason ASCII 1967 replaced ← with _ and ↑ with ^ (explicitly still “circumflex
accent” in Unicode) and added ` (“grave accent”, likewise).
ANSI made this optional in the 1986 revision, in §2.1.2 — “The use of BS for forming composite characters is not required.” — with a note that it would likely be removed from a future revision (but there never was another one).
> In a more common example, ASCII has nothing to say about how you move the cursor to the start of a new line.
It does; it just says something slightly unfortunate about code 0x0A.
CR Carriage Return
A format effector which moves the active position
to the first character position *on the same line*.
LF Line Feed
A format effector which advances the active position
to the *same character position* of the next line.
[Italics added] But then it says:
The Format Effectors are intended for equipment in
which horizontal and vertical movements are effected
separately. If equipment requires the action of
CARRIAGE RETURN to be comhined with a vertical movement,
the Format Effector for that vertical movement
may be used to effect the combined movement. For example,
if NEW LINE (symbol NL, equivalent to CR + LF)
is required, FE2 shall be used to represent it. This
substitution requires agreement between the sender and
the recipient of the data.
The use of these combined functions may be restricted
for international transmission on general switched telecommunication
networks (telegraph and telephone networks).
So CR LF will unambiguously get you the first position on the next line. The code for LF is allowed to be replaced by NL by “agreement”, but CR can't move to the next line.