The inset 1px border shadow draws a 1-pixel inset stroke at a specified color — visually a border, technically a shadow. Linear's entire elevation system relies on this primitive (`box-shadow: rgb(35, 37, 42) 0 0 0 1px inset` on cards). The advantage over `border` is that inset shadows don't affect layout dimensions and stack cleanly with focus-visible accent rings. Common in modern dark-first design systems (Vercel, Cursor, Warp).
Glossary · elevation
Inset 1px Border Shadow
A CSS box-shadow technique (`box-shadow: <color> 0 0 0 1px inset`) used for surface delimitation without traditional drop shadow.
Catalogued examples
2 catalogued sites demonstrate inset 1px border shadow in production.