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: