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

import { walk, generate, keyword as resolveKeyword } from 'css-tree';
import createDeclarationIndexer from './createDeclarationIndexer.js';
import processSelector from './processSelector.js';

export default function prepare(ast, options) {
    const markDeclaration = createDeclarationIndexer();

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

    walk(ast, {
        visit: 'Atrule',
        enter(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.forEach(function(rule) {
                    rule.prelude.children.forEach(function(simpleselector) {
                        simpleselector.compareMarker = simpleselector.id;
                    });
                });
            }
        }
    });

    return {
        declaration: markDeclaration
    };
};