Right now, if you want a border/line between Flex items or Grid cells, you need hacks like * + * { border-top: 1px solid #ccc; } or more complex to handle multiple rows and columns together. These hacks fall apart very quickly, for instance if the first item has display: none. It would be nice to add a property explicitly for this, and there is one that currently works in multi-column layout: column-rule. Note that the related property column-gap already works in Flex and Grid, along with row-gap and the gap shorthand. I propose adding row-rule and rule as well.