The border-inline-start CSS property specifies the border on the start side of the inline axis using logical box terminology. Rather than tying the rule to a physical side like left or right, it applies to whichever edge is considered the inline “start” for that element. As a longhand logical border property it determines the visible border on that inline-start edge and can be used instead of or alongside shorthands to control that one side precisely.

Which physical edge gets styled by border-inline-start depends on the document’s inline direction and writing mode: the mapping is resolved based on the element’s writing-mode and direction settings. For example, in a horizontal writing mode with left-to-right text, the inline start is the left edge; with right-to-left text it becomes the right edge. In vertical writing modes the inline axis changes orientation, so the inline-start edge will map to a different physical side (top or bottom) depending on the specific mode and direction.

Using border-inline-start is especially useful for internationalized layouts because it allows the same stylesheet to automatically flip borders when text direction or writing mode changes, avoiding separate rules for physical sides like border-left and border-right. It integrates with the rest of the border subsystem: you can control the inline-start edge’s appearance in concert with broader shorthands such as border-inline or border, and the final rendered border depends on the effective values of the related longhands border-style, border-width, and border-color.

In practice, prefer logical properties like border-inline-start when building components that must work across locales and writing modes, and when you want your layout to flip naturally without additional rules. Keep in mind cascade and specificity rules still apply: side-specific properties override broader shorthands when they conflict, and borders won’t render unless an appropriate border style and width are in effect. Use this property where a single logical-edge border is required, and combine it with other logical box properties for consistent, direction-aware layouts.