Wait, '1/3' isn't a fraction, it's a range from 1 to 3? Why in God's name would they choose that syntax? Ranges have plenty of reasonable, well known syntaxes in programming. E.g. 1..3
A lot of the problems programmers seem to have with CSS probably stem from this misapprehension.
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...