Do you have a reference for this particular usage of slash? In print, “x / y” is often used to mean “item x out of y” [0], for example in page numbers. I’m only familiar with using slashes for ranges in ISO 8601, where it has purely syntactic reasons and (AFAICT) doesn’t adopt any preexisting convention.

I imagine the slash in CSS grid is for similar syntactic reasons, because the syntax also admits hyphenated identifiers [1] (custom-ident), which precludes the use of “-”. (They could have used a range operator like “~” or “..” though.)

[0] https://en.wikipedia.org/wiki/Slash_(punctuation)#Numbering

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/grid-column...

And that is exactly what 1/3 means in CSS Grids, the element occupies the grid from slot 1 to 3.

“1 of 3” is not the same thing as “1 to 3”, so I don’t think they mean the same thing here.

As far as I know, 1/3 means "one of three" in print - which actually is division ("1 of 3" means the subject's position is 1/3 of the way through).

CSS uses "/" for division (fractions) inside `calc`, so I don't think not understanding the language of "print" is where the frustration comes from nowadays. I think CSS is also part programming language now.