node_modules/csso/lib/restructure/prepare/specificity.js

module.exports = function specificity(simpleSelector) {
    var A = 0;
    var B = 0;
    var C = 0;

    simpleSelector.children.each(function walk(node) {
        switch (node.type) {
            case 'SelectorList':
            case 'Selector':
                node.children.each(walk);
                break;

            case 'IdSelector':
                A++;
                break;

            case 'ClassSelector':
            case 'AttributeSelector':
                B++;
                break;

            case 'PseudoClassSelector':
                switch (node.name.toLowerCase()) {
                    case 'not':
                        node.children.each(walk);
                        break;

                    case 'before':
                    case 'after':
                    case 'first-line':
                    case 'first-letter':
                        C++;
                        break;

                    // TODO: support for :nth-*(.. of <SelectorList>), :matches(), :has()

                    default:
                        B++;
                }
                break;

            case 'PseudoElementSelector':
                C++;
                break;

            case 'TypeSelector':
                // ignore universal selector
                if (node.name.charAt(node.name.length - 1) !== '*') {
                    C++;
                }
                break;

        }
    });

    return [A, B, C];
};