css-loader.js

import path from 'path';

function shouldApply(url) {
  return path.extname(url) === '.css';
}

export function getFormat(url, context, defaultGetFormat) {
  if (shouldApply(url)) {
    return {
      format: 'module',
    };
  }
  return defaultGetFormat(url, context, defaultGetFormat);
}

export async function transformSource(source, context, defaultTransformSource) {
  if (shouldApply(context.url)) {
    return { source: '' };
  }

  return defaultTransformSource(source, context, defaultTransformSource);
}

demo.js

import './styles.css'

console.log('works!');

package.json

{
  "name": "css-node-loader",
  "type": "module",
  "version": "1.0.0",
  "exports": {
    ".": "./css-loader.js"
  },
  "scripts": {
    "test": "node --loader ./css-loader.js demo.js"
  }
}

styles.css

.highlight {
  color: blue;
}