In CSS Flexbox, why are there no "justify-items" and "justify-self" properties?

Originally, Flexbox did not include justify-items or justify-self because item-level justification was intended to be handled by the container (justify-content) and by per-item properties like margin: auto. Meanwhile, Grid Layout introduced justify-items and justify-self for more granular control. Flexbox’s older spec preceded the modern “Box Alignment” properties, so justify-items/justify-self were simply not part of Flexbox’s initial design.

That said, the newer CSS Box Alignment module aims to unify alignment properties across layout models, so some browsers now support justify-self for flex items in certain conditions. But historically and in many current practical scenarios, flex items rely on container-level alignment (justify-content) and item-level hacks like margin: auto for horizontal alignment.

1. Historical Context

1. Flexbox (2012–2017)
   When Flexbox was standardized, it provided:

   • justify-content: space distribution on the main axis
   • align-items and align-content: alignment on the cross axis (and distribution for multi-line flex containers)
   • align-self: item-level cross-axis alignment

   The spec did not define justify-items or justify-self. Instead, flex item alignment on the main axis is typically handled collectively by the container (justify-content) or individually by setting margin: auto on a flex item.

2. Grid Layout (2017–2020)
   The CSS Grid spec introduced justify-items and justify-self to control horizontal alignment of grid items, complementing align-items/align-self. Grids needed more granular control because they can have multiple columns and rows simultaneously, and each cell might need unique alignment rules.

3. Box Alignment Module
   Subsequent CSS specs (the Box Alignment module) define a unified set of alignment properties (e.g., justify-self, justify-items, align-self, align-items) for different layout modes (Block, Flex, Grid). However, not all properties are fully implemented or have consistent behavior in Flexbox across browsers.

2. How Flexbox Handles “Self” Justification

In flex layouts, a single item can align itself along the main axis in one of two typical ways:

1. Margin Auto Trick
   If you want to push a single item to one side (or center) while others do not move, you can use:

   .flex-item {
     margin-left: auto; /* pushes item to the right in a left-to-right container */
     /* or margin: auto if you want to center it in both axes (assuming 
        align-items: center for the cross axis) */
   }

   This is a well-known workaround that effectively “justifies” an individual flex item differently than its siblings.

2. Align-Self for Cross Axis
   Flexbox already has align-self for the cross axis, but on the main axis, justify-content generally applies to all items collectively.

3. Will justify-self/justify-items Ever Work in Flexbox?

Current Behavior

• Some browsers do partially support justify-self on flex items because of the Box Alignment module. For example, Safari (and certain Chrome versions behind flags) might allow justify-self: center; on a flex item, but it’s not consistently implemented across all browsers in the same way.
• Even where recognized, justify-self in a flex context might just behave like margin: auto, or be overshadowed by how justify-content controls distribution.

Future Outlook

• The CSS Working Group aims for consistent alignment properties across all layout models, so eventually we may see broader official or consistent support for justify-self in flex contexts. But for now, it’s safer to rely on well-supported features (like margin: auto) to individually align flex items along the main axis.

4. Practical Workarounds

1. Use margin: auto
   If you want one item to be centered or aligned differently:

   .container {
     display: flex;
     justify-content: flex-start; /* or some default */
   }
   .item.special {
     margin-left: auto;
   }

2. Wrap the item if needed
   If an individual item really needs distinct alignment and you can’t use margin trick, you might wrap it in another container that either uses a separate flex or alignment rule.

3. Consider CSS Grid
   If your layout requires fine-grained per-item justification, Grid might be more suitable. Grid has justify-self, align-self, justify-items, and align-items, all fully recognized.

5. Summary

• Flexbox was designed with collective alignment (justify-content) along the main axis, rather than each item individually.
• justify-items and justify-self exist in CSS Grid (and eventually in the unified Box Alignment spec), but they were not part of Flexbox’s initial approach and remain inconsistently implemented.
• Workaround: In day-to-day practice, margin: auto is the standard way to shift a single flex item’s main-axis alignment.

Ultimately, Flexbox focuses on distributing space among items at a container-wide level, whereas CSS Grid (and the future Box Alignment spec) is more granular, supporting item-level justification out of the box.