If you take a selector like .class-a :where(.class-b, .class-c) and try prefixing it with .prefix-class, it results in a selector .prefix-class .class-a :where(.class-b, .prefix-class .class-c), which is incorrect. I think the expected result should be .prefix-class .class-a :where(.class-b, .class-c)

It appears to be because the code splits a selector string using the string .split( ',' ) method:

I think the AST does provide an already split cssRule.selectors property, so splitting the string might not be needed, it may be ok to lean on what the AST provides.