Viewport <#document> at (0,0) content-size 800x600 children: not-inline
  BlockContainer <html> at (1,1) content-size 798x176 [BFC] children: not-inline
    BlockContainer <body> at (10,10) content-size 780x158 children: not-inline
      Box <div.flexbox> at (11,11) content-size 200x156 flex-container(column) [FFC] children: not-inline
        BlockContainer <div> at (12,12) content-size 50x50 flex-item [BFC] children: not-inline
        BlockContainer <div> at (12,64) content-size 50x50 flex-item [BFC] children: not-inline
        BlockContainer <div> at (12,116) content-size 50x50 flex-item [BFC] children: not-inline

ViewportPaintable (Viewport<#document>) [0,0 800x600]
  PaintableWithLines (BlockContainer<HTML>) [0,0 800x178]
    PaintableWithLines (BlockContainer<BODY>) [9,9 782x160]
      PaintableBox (Box<DIV>.flexbox) [10,10 202x158]
        PaintableWithLines (BlockContainer<DIV>) [11,11 52x52]
        PaintableWithLines (BlockContainer<DIV>) [11,63 52x52]
        PaintableWithLines (BlockContainer<DIV>) [11,115 52x52]
