Please enclose the 'content' element in quotation marks.

NG

function HelloWorld() {
  return (
    <div
      className={css({
        _before: {
          content: "Hello, world!" // <--- NG
        }
      })}
    ></div>
  )
}

You might encounter the bellow error message in a Developper tools in Chrome...

Determines which page-based occurrence of a given element is applied to a counter or string value.

OK

function HelloWorld() {
  return (
    <div
      className={css({
        _before: {
          content: '"Hello, world!"' // <--- OK
        }
      })}
    ></div>
  )
}