[Metaboat]

104 :nth-child(3n){list-style:"Fizz"}:nth-child(5n){list-style:"Buzz"}:nth-child(15n){list-style:"FizzBuzz"}

data:text/html,<style>:nth-child(3n){list-style:"Fizz"}:nth-child(5n){list-style:"Buzz"}:nth-child(15n){list-style:"FizzBuzz"}</style><ol id=o><script>o.innerHTML='<li>'.repeat(99)</script>

[cefqrn]

89: :nth-child(3n)::marker{content:"Fizz"var(--b,)}:nth-child(5n){list-style:"Buzz";--b:"Buzz

with help from @bulmenisaurus

[Metaboat]

103 :nth-child(3n){list-style:"Fizz"}:nth-child(5n){list-style:"Buzz"}:nth-child(15n){list-style:"FizzBuzz"

[Metaboat]

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

[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

[Metaboat]

thank you sir! i can improve the html op css was 152, we have the whole page at 144

data:text/html,<ol id=o><script>o.innerHTML='<li>'.repeat(100)</script><style>:nth-child(3n){list-style:"Fizz"}:nth-child(5n):after{content:"Buzz