Earlier quoted context omitted.
>The latter is what typewriters do for the Return key. Minor correction: mechanical typewriters do not have a Return key, but they have both operations (line feed, as well as carriage return). The carriage return lever is typically rigged to also do line feed at the same time, by a preset amount of lines (which can be set to 0), or you can push the carriage without engaging line feed. Technically, the lever would do…
They didn't say "mechanical typewriters", just "typewriters". Electric typewriters did have a Return key that did work the way they described.
1) "Typewriters" in parent's comment didn't refer to mechanical typewriters, but
2) Line feed/carriage return semantics, as well as the UX of combining them into one action to proceed to the next line of text, predate electric typewriters and were effectively the same on mechanical ones.
As I wrote in the other comment, the subtle difference in semantics comes from teletypes, which couldn't advance the paper feed and return the carriage fast enough to print the next character in the timespan of one command.
Not that it applied to all teletypes, but it was the case for a very popular unit.
The makers of that machine deliberately didn't include a single command that would do CR/LF so that there'd be no way for the users to notice that.
The ordering, CR then LF, differs from the one on mechanical typewriters, where LF always precedes CR when you use the big lever, allowing one to use the same lever to produce blank lines without moving the carriage (in effect, doing LF LF LF ... LF CR).
On the teletypes though, CR LF ordering was, in any case, a lie, since in actuality, LF was happening somewhere in the middle of the carriage return, which took the time span of two commands.
The CR command had to precede LF on the teletype because it took longer, but since the mechanisms were independent, they could be executed at the same time.
This is the difference from mechanical typewriters.
Typing mechanism was also independent of CR and LF, and running CR + [type character] at the same time was bad. But having fixed-time-per-command simplified everything, so instead of waiting (..which means buffering - with potential overflow issues - or a protocol to tell the sending party to wait, which is a lot more complex), hacks like this were put in place.
My IBM selectric is not functional (got it as a repair project, didn't get to do it yet), so I can't verify, but I'd guess it doesn't need to do CR then LF, since it can simply not process input while the carriage is returning. It's OK for it to do CR and LF in any order, or simultaneously.
If the operator presses and releases a button during this time, the machine can simply do nothing; the operator will re-type the character the next instant, using the buffer in their head where the text ultimately comes from.
The teletypes didn't have that luxury, as the operator on the other end could be a computer, which was told it could send output at a certain rate, and by golly it did. Not processing a command would mean dropping data.
All that is to say that CR and LF are present on both typewriters and teletypes, with the following differences:
* mechanical typewriters always do LFCR due the mechanics of the carriage return lever, which was designed for a human operator;
* teletypes do CRLF because that's how they cope with the typist being a machine that can't be told to wait a bit until the carriage returns;
* and electric typewriters are somewhere in betwen and could do whatever, because the CR lever was replaced by the motor (like on a teletype), but the operator was still a human that could wait half a second without forgetting what it is that they wanted to type.
IMO, it's worth keeping CRLF around simply because it's a part of computer and technology history that spans nearly two centuries, from typewriters to Google docs.