In particular, we should be able to support generating CSS files that:

• Order CSS correctly but don't use @layer or a polyfill.
  • CSS layers aren't needed for many use cases.
• Use native @layer.
  • Useful for explicitly controlling order, including when generating multiple sheets.
• Polyfill @layer.
  • If layers are needed but browser support is not adequate.
  • Use PostCSS instead of rolling our own solution: https://www.npmjs.com/package/@csstools/postcss-cascade-layers