The border-collapse property sets how to display borders around table cells.
This property comes into play when a border is set for the cells, then a line of double thickness will be obtained at the junction of the cells. The collapse value causes the browser to analyze such places in the table and remove double lines in it. In this case, only one border remains between the cells, which simultaneously belongs to both cells. The same rule holds for outer borders, when a frame is added around the table itself.
border-collapse: collapse | separate;