const parser = require('postcss-selector-parser');
const transform = selectors => {
    selectors.walk(selector => {
        // do something with the selector
        console.log(String(selector))
    });
};

const transformed = parser(transform).processSync('h1, h2, h3');

const parser = require('postcss-selector-parser');
const normalized = parser().processSync('h1, h2, h3', {lossless: false});
// -> h1,h2,h3