li:nth-child(3n), li:nth-child(5n) { list-style: none }
  li:nth-child(3n)::before { content: "Fizz" }
  li:nth-child(5n)::after { content: "Buzz" }

  $ curl -sS https://susam.net/code/web/css-fizz-buzz-ol.html | sed -n '/none/,/after/p' |  tr -d '[:space:]' 
  li:nth-child(3n),li:nth-child(5n){list-style:none}li:nth-child(3n)::before{content:"Fizz"}li:nth-child(5n)::after{content:"Buzz"}
  $ curl -sS https://susam.net/code/web/css-fizz-buzz-ol.html | sed -n '/none/,/after/p' |  tr -d '[:space:]' | wc -c
  129

<style> p{counter-increment:n} p:not(:nth-child(5n)):before{content:counter(n)} p:nth-child(3n):before{content:"Fizz"} p:nth-child(5n):after{content:"Buzz"} </style><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p>

https://codepen.io/dsmmcken/pen/WbwYOEQ?editors=0100

p{counter-increment:n;--n:counter(n)}p:nth-child(3n){--f:"Fizz"}p:nth-child(5n){--b:"Buzz";--n:''}p::after{content:var(--f,var(--n))var(--b,'')}

Now, arguably it's not a "number or word that appears in the output", that's true, but it is a part of the logic of FizzBuzz. It's the fact that there are 100 and exactly 100 <li> elements in the HTML part.

I mean, it wouldn't be a correct FizzBuzz without precisely that HTML. Having exactly 100 <li> elements is implementing the part of the logic that a. loops, and b. stops at 100.

But of course this is just nitpicking, as I said ;)

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>