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

var resolveKeyword = require('css-tree').keyword;
var walk = require('css-tree').walk;
var generate = require('css-tree').generate;
var createDeclarationIndexer = require('./createDeclarationIndexer');
var processSelector = require('./processSelector');

module.exports = function prepare(ast, options) {
    var markDeclaration = createDeclarationIndexer();

    walk(ast, {
        visit: 'Rule',
        enter: function processRule(node) {
            node.block.children.each(markDeclaration);
            processSelector(node, options.usage);
        }
    });

    walk(ast, {
        visit: 'Atrule',
        enter: function(node) {
            if (node.prelude) {
                node.prelude.id = null; // pre-init property to avoid multiple hidden class for generate
                node.prelude.id = generate(node.prelude);
            }

            // compare keyframe selectors by its values
            // NOTE: still no clarification about problems with keyframes selector grouping (issue #197)
            if (resolveKeyword(node.name).basename === 'keyframes') {
                node.block.avoidRulesMerge = true;  /* probably we don't need to prevent those merges for @keyframes
                                                       TODO: need to be checked */
                node.block.children.each(function(rule) {
                    rule.prelude.children.each(function(simpleselector) {
                        simpleselector.compareMarker = simpleselector.id;
                    });
                });
            }
        }
    });

    return {
        declaration: markDeclaration
    };
};