Created by gist-it (https://github.com/egoist/gist-it)

input.js

import './a.css'

out.js

(function () {
'use strict';

function styleInject(css, ref) {
  if ( ref === void 0 ) ref = {};
  var insertAt = ref.insertAt;

  if (!css || typeof document === 'undefined') { return; }

  var head = document.head || document.getElementsByTagName('head')[0];
  var style = document.createElement('style');
  style.type = 'text/css';

  if (insertAt === 'top') {
    if (head.firstChild) {
      head.insertBefore(style, head.firstChild);
    } else {
      head.appendChild(style);
    }
  } else {
    head.appendChild(style);
  }

  if (style.styleSheet) {
    style.styleSheet.cssText = css;
  } else {
    style.appendChild(document.createTextNode(css));
  }
}

var css = "body {\n  color: red;\n}\n";
styleInject(css);

}());

package.json

{
  "name": "test",
  "version": "1.0.0",
  "license": "MIT",
  "devDependencies": {
    "rollup": "^0.56.3",
    "rollup-plugin-commonjs": "^8.3.0",
    "rollup-plugin-node-resolve": "^3.0.3",
    "rollup-plugin-postcss": "^1.3.1"
  }
}

rollup.config.js

export default {
  input: './input.js',
  output: {
    file: 'out.js',
    format: 'iife'
  },
  plugins: [
    require('rollup-plugin-node-resolve')({ jsnext: true, main: true, browser: true }),
    require('rollup-plugin-commonjs')(),
    require('rollup-plugin-postcss')()
  ]
}