layer at (0,0) size 800x600
  RenderCanvas at (0,0) size 800x600
layer at (0,0) size 800x600
  RenderBlock {HTML} at (0,0) size 800x600
    RenderBody {BODY} at (8,8) size 784x584 [border: (2px solid #FF0000)]
      RenderBR {BR} at (14,14) size 0x28
      RenderText {TEXT} at (14,42) size 36x28
        text run at (14,42) width 36: "xxx"
      RenderBR {BR} at (0,0) size 0x0
selection is CARET:
start:      position 3 of child 3 {TEXT} of root {BODY}
upstream:   position 3 of child 3 {TEXT} of root {BODY}
downstream: position 0 of child 4 {BR} of root {BODY}