Most people never see the panel borders when they read the daily funnies. The borders are just part of the visual language of sequential art. Like paragraph indentations, they provide visual clues as to how the material is organized, but are only processed unconsciously.
Mark Tatulli , in his strip Lio, has had some fun playing with panel borders before (Lio's Comic Cameos 2, Lio and the Fourth Wall). It's always something of a surprise when what was intended to be part of the background suddenly comes to the fore. No need to explain this one, but there's something to think about. Based on Tatulli's depiction, what is the nature of the panel border? (click on image to enlarge)