border-collapse property is for use on
<table> elements (or elements made to behave like a table through
display: table or
display: inline-table). There are two values:
separate(default) - in which all table cells have their own independent borders and there may be space between those cells as well.
collapse- in which both the space and the borders between table cells collapse so there is only one border and no space between cells.
collapse, it is notable that properties like
border-radius (on actual borders) don't do anything. You'll need
border-collapse: separate; if you need either of those things.
Here's an example where you can swap between the two: