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 784x17 children: not-inline
      BlockContainer <details> at (8,8) content-size 784x17 children: not-inline
        ListItemBox <summary> at (32,8) content-size 760x17 children: inline
          frag 0 from TextNode start: 0, length: 13, rect: [32,8 114.625x17] baseline: 13.296875
              "I'm a summary"
          TextNode <#text>
          ListItemMarkerBox <(anonymous)> at (8,8) content-size 12x17 children: not-inline
        BlockContainer <slot> at (8,25) content-size 784x0 children: not-inline
      BlockContainer <(anonymous)> at (8,25) content-size 784x0 children: inline
        TextNode <#text>

ViewportPaintable (Viewport<#document>) [0,0 800x600]
  PaintableWithLines (BlockContainer<HTML>) [0,0 800x600]
    PaintableWithLines (BlockContainer<BODY>) [8,8 784x17]
      PaintableWithLines (BlockContainer<DETAILS>) [8,8 784x17]
        PaintableWithLines (ListItemBox<SUMMARY>) [32,8 760x17]
          TextPaintable (TextNode<#text>)
          MarkerPaintable (ListItemMarkerBox(anonymous)) [8,8 12x17]
        PaintableWithLines (BlockContainer<SLOT>) [8,25 784x0]
      PaintableWithLines (BlockContainer(anonymous)) [8,25 784x0]
