new_contents: # coding: utf8
  """
      weasyprint.stacking
      -------------------
  
      :copyright: Copyright 2011-2012 Simon Sapin and contributors, see AUTHORS.
      :license: BSD, see LICENSE for details.
  
  """
  
  from __future__ import division, unicode_literals
  
  from .formatting_structure import boxes
  
  
  def establishes_stacking_context(box):
      return (
          box.style.position != 'static' and box.style.z_index != 'auto'
      ) or (
          box.style.opacity < 1
      ) or (
          box.style.transform  # empty list for 'transform: none'
      )
  
  
  class StackingContext(object):
      def __init__(self, box):
          self.negative_z_contexts = []  # 3: Child contexts, z-index < 0
          self.block_boxes = []  # 4, 7: In flow, non positioned
          self.float_boxes = []  # 5: Non positioned
          self.zero_z_contexts = []  # 8: Child contexts, z-index = 0
          self.positive_z_contexts = []  # 9: Child contexts, z-index > 0
  
          self.box = self._dispatch_children(box)
          self.z_index = box.style.z_index
          if self.z_index == 'auto':
              self.z_index = 0
  
      def _dispatch_children(self, box):
          if not isinstance(box, boxes.ParentBox):
              return box
  
          children = []
          for child in box.children:
              if establishes_stacking_context(child):
                  context = StackingContext(child)
                  if context.z_index < 0:
                      self.negative_z_contexts.append(context)
                  elif context.z_index == 0:
                      self.zero_z_contexts.append(context)
                  elif context.z_index > 0:
                      self.positive_z_contexts.append(context)
                  # Remove from children
              else:
                  child = self._dispatch_children(child)
                  if child.style.position != 'static':
                      assert child.style.z_index == 'auto'
                      # sub-contexts are already removed
                      context = StackingContext(child)
                      self.zero_z_contexts.append(context)
                  elif child.is_floated():
                      self.float_boxes.append(child)
                  elif isinstance(child, boxes.BlockBox):
                      self.block_boxes.append(child)
  
                  children.append(child)
          return box.copy_with_children(children)
                  
subject: Add a StackingContext in preparation for z-index drawing.