[Metaboat]

95 :nth-child(5n){list-style:}:nth-child(3n){list-style:"Fizz"}:nth-child(5n)::after{content:"Buzz

[Metaboat]

68 :nth-child(3n){list-style:"Fizz"}:nth-child(5n)::after{content:"Buzz

[bulmenisaurus]

This version fails for numbers like 5, showing "5. Buzz". In the case of a number divisible by 5 but not 3 nothing prevents the 5 in the ::marker from rendering.

Edit: it looks like your version can be fixed into a working 85:

  :nth-child(5n){list-style:'';&:after{content:"Buzz"}}:nth-child(3n){list-style:"Fizz"

[eurleif]

67 :nth-child(3n){list-style:"Fizz"}:nth-child(5n):after{content:"Buzz