[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"