code: function walkCss(node, fn){
      node.rules.forEach(function(rule){
          // @media etc
          if (rule.rules) {
              walkCss(rule, fn);
  
              return;
          }
  
          // keyframes
          if (rule.keyframes) {
              rule.keyframes.forEach(function(keyframe){
                  if (keyframe.type === 'keyframe') {
                      fn(keyframe.declarations, rule);
                  }
              });
  
              return;
          }
  
          // @charset, @import etc
          if (!rule.declarations) {
              return;
          }
  
          fn(rule.declarations, node);
      });
  }
docs: Functions ============================================================================= 
  Visit `node` declarations recursively and invoke `fn(declarations, node)`.
  
  @param {object} node
  @param {function} fn