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

with help from @bulmenisaurus

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

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

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

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

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"

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

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

Your code is showing the numbers 5, 10, 20. that's not correct, is it?

Yours prints 1. 2. Fizz 4. 5. Buzz Fizz 7.

But it should be 1. 2. Fizz 4. Buzz Fizz 7.