Just pointing out that this is merely an expectation and not a rule. You can happily design a "monospace" font in any font publishing software that blatantly disrespects a single width. In fact, you can just manually make any letterform 2x width, and that is exactly what most monospace ligatures do! (except the bad ones, which combine e.g. "fi" ligature a single width letter)
The ligature letters are just... 2x width. Believe it or not this is totally legal!
https://www.hanselman.com/blog/monospaced-programming-fonts-...
> you can just manually make any letterform 2x width,
Yes. Any glyph may be wider than its advance. The advance, however, must remain the same across all glyphs. If it does not, the font is not monospaced.
The passage you’ve quoted refers not to the width, but to the advance.
Well, the code text probably, in most programming languages. But code also contains comments, and comments are very often written in other languages than English in personal project and/or during development phases before clean up for merging, and sometimes even after that.