index.js

const CSS = require('css');

function scopeCSS(css, scope) {
  const ast = CSS.parse(css);
  for (let rule of ast.stylesheet.rules) {
    if (rule.type == 'rule') {
      rule.selectors = rule.selectors.map(selector => `${scope} ${selector}`);
    }
  }
  return CSS.stringify(ast);
}

const orig = 'div { color: black; } p, a, div > span { font-size: 20px; }';
const rewritten = scopeCSS(orig, '#foo');

const pre = document.createElement('pre');

pre.innerText =
`/* ORIGINAL */
${orig}

/* REWRITTEN */
${rewritten}`;

document.body.appendChild(pre);

package.json

{
  "name": "requirebin-sketch",
  "version": "1.0.0",
  "dependencies": {
    "css": "2.2.1"
  }
}