thanks! i should have guessed that it would be kitty
as for plain text, specifically what i want is plain text that can accommodate proportional fonts and different font sizes without losing the ability to do layout. actually specifying things like fonts, font sizes, colors, padding, margins, borders, corner radii, drop shadows, filters, etc., might be done not at all, with some kind of standoff markup (where the markup is in a different file), with embedded escape sequences, or with syntax-highlighting-like rules
^k and ^l, vt and ff, have an advantage over other possible delimiter characters: they are conventionally considered whitespace, making them legal in between tokens in most existing programming languages
with respect to what control characters have historically been used for, i can recommend erica fischer's paper on the history of ascii: https://ia801805.us.archive.org/24/items/enf-ascii/ascii.pdf as well as tom jennings's history of character codes
as for plain text, specifically what i want is plain text that can accommodate proportional fonts and different font sizes without losing the ability to do layout. actually specifying things like fonts, font sizes, colors, padding, margins, borders, corner radii, drop shadows, filters, etc., might be done not at all, with some kind of standoff markup (where the markup is in a different file), with embedded escape sequences, or with syntax-highlighting-like rules
^k and ^l, vt and ff, have an advantage over other possible delimiter characters: they are conventionally considered whitespace, making them legal in between tokens in most existing programming languages
with respect to what control characters have historically been used for, i can recommend erica fischer's paper on the history of ascii: https://ia801805.us.archive.org/24/items/enf-ascii/ascii.pdf as well as tom jennings's history of character codes