Viewport <#document> at (0,0) content-size 800x600 children: not-inline
  BlockContainer <html> at (0,0) content-size 800x600 [BFC] children: not-inline
    BlockContainer <body> at (8,8) content-size 784x300 children: not-inline
      BlockContainer <input.btn> at (13,10) content-size 290x296 children: not-inline
        BlockContainer <(anonymous)> at (13,10) content-size 290x296 flex-container(column) [FFC] children: not-inline
          BlockContainer <(anonymous)> at (13,150) content-size 290x16 flex-item [BFC] children: inline
            frag 0 from TextNode start: 0, length: 31, rect: [37,150 242.4375x16] baseline: 12.5
                "Should be located in the center"
            TextNode <#text>
      BlockContainer <(anonymous)> at (8,308) content-size 784x0 children: inline
        TextNode <#text>

ViewportPaintable (Viewport<#document>) [0,0 800x600]
  PaintableWithLines (BlockContainer<HTML>) [0,0 800x600]
    PaintableWithLines (BlockContainer<BODY>) [8,8 784x300]
      PaintableWithLines (BlockContainer<INPUT>.btn) [8,8 300x300]
        PaintableWithLines (BlockContainer(anonymous)) [13,10 290x296]
          PaintableWithLines (BlockContainer(anonymous)) [13,150 290x16]
            TextPaintable (TextNode<#text>)
      PaintableWithLines (BlockContainer(anonymous)) [8,308 784x0]
