The effect of this property is dependent of the layout mode we are in:

• In block-level layouts, it aligns the items inside their containing block on the inline axis.
• For absolutely-positioned elements, it aligns the items inside their containing block on the inline axis, accounting for the offset values of top, left, bottom, and right.
• In table cell layouts, this property is ignored (more about alignment in block, absolute positioned and table layout)
• In flexbox layouts, this property is ignored (more about alignment in Flexbox)
• In grid layouts, it aligns the items inside their grid areas on the inline axis (more about alignment in grid layouts)

/* Basic keywords */
justify-items: normal;
justify-items: stretch;

/* Positional alignment */
justify-items: center; /* Pack items around the center */
justify-items: start; /* Pack items from the start */
justify-items: end; /* Pack items from the end */
justify-items: flex-start; /* Equivalent to 'start'. Note that justify-items is ignored in Flexbox layouts. */
justify-items: flex-end; /* Equivalent to 'end'. Note that justify-items is ignored in Flexbox layouts. */
justify-items: self-start;
justify-items: self-end;
justify-items: left; /* Pack items from the left */
justify-items: right; /* Pack items from the right */

/* Baseline alignment */
justify-items: baseline;
justify-items: first baseline;
justify-items: last baseline;

/* Overflow alignment (for positional alignment only) */
justify-items: safe center;
justify-items: unsafe center;

/* Legacy alignment */
justify-items: legacy right;
justify-items: legacy left;
justify-items: legacy center;

/* Global values */
justify-items: inherit;
justify-items: initial;
justify-items: revert;
justify-items: revert-layer;
justify-items: unset;

This property can take one of four different forms:

• Basic keywords: one of the keyword values normal or stretch.
• Baseline alignment: the baseline keyword, plus optionally one of first or last.
• Positional alignment: one of: center, start, end, flex-start, flex-end, self-start, self-end, left, or right. Plus optionally safe or unsafe.
• Legacy alignment: the legacy keyword, followed by one of left or right.

normal

The effect of this keyword is dependent of the layout mode we are in:

• In block-level layouts, the keyword is a synonym of start.
• In absolutely-positioned layouts, the keyword behaved like start on replaced absolutely-positioned boxes, and as stretch on all other absolutely-positioned boxes.
• In table cell layouts, this keyword has no meaning as this property is ignored.
• In flexbox layouts, this keyword has no meaning as this property is ignored.
• In grid layouts, this keyword leads to a behavior similar to the one of stretch, except for boxes with an aspect ratio or an intrinsic sizes where it behaves like start.

start

The item is packed flush to each other toward the start edge of the alignment container in the appropriate axis.

end

The item is packed flush to each other toward the end edge of the alignment container in the appropriate axis.

flex-start

For items that are not children of a flex container, this value is treated like start.

flex-end

For items that are not children of a flex container, this value is treated like end.

self-start

The item is packed flush to the edge of the alignment container of the start side of the item, in the appropriate axis.

self-end

The item is packed flush to the edge of the alignment container of the end side of the item, in the appropriate axis.

center

The items are packed flush to each other toward the center of the alignment container.

left

The items are packed flush to each other toward the left edge of the alignment container. If the property's axis is not parallel with the inline axis, this value behaves like start.

right

The items are packed flush to each other toward the right edge of the alignment container in the appropriate axis. If the property's axis is not parallel with the inline axis, this value behaves like start.

baseline, first baseline, last baseline

Specifies participation in first- or last-baseline alignment: aligns the alignment baseline of the box's first or last baseline set with the corresponding baseline in the shared first or last baseline set of all the boxes in its baseline-sharing group. The fallback alignment for first baseline is start, the one for last baseline is end.

stretch

If the combined size of the items is less than the size of the alignment container, any auto-sized items have their size increased equally (not proportionally), while still respecting the constraints imposed by max-height/max-width (or equivalent functionality), so that the combined size exactly fills the alignment container.

safe

If the size of the item overflows the alignment container, the item is instead aligned as if the alignment mode were start.

unsafe

Regardless of the relative sizes of the item and alignment container, the given alignment value is honored.

legacy

Makes the value inherited by the box descendants. Note that if a descendant has a justify-self: auto value, the legacy keyword is not considered by the descend, only the left, right, or center value associated to it.

justify-items = 
  normal                                              |
  stretch                                             |
  <baseline-position>                                 |
  <overflow-position>? [ <self-position> | left | right ]  |
  legacy                                              |
  legacy && [ left | right | center ]                 

<baseline-position> = 
  [ first | last ]?  &&
  baseline           

<overflow-position> = 
  unsafe  |
  safe    

<self-position> = 
  center      |
  start       |
  end         |
  self-start  |
  self-end    |
  flex-start  |
  flex-end    

In the following example we have a simple 2 x 2 grid layout. Initially the grid container is given a justify-items value of stretch (the default), which causes the grid items to stretch across the entire width of their cells.

If you hover or tab onto the grid container however, it is given a justify-items value of center, which causes the grid items to span only as wide as their content width, and align in the center of their cells.

HTML

<article class="container" tabindex="0">
  <span>First child</span>
  <span>Second child</span>
  <span>Third child</span>
  <span>Fourth child</span>
</article>

CSS

html {
  font-family: helvetica, arial, sans-serif;
  letter-spacing: 1px;
}

article {
  background-color: red;
  display: grid;
  grid-template-columns: 1fr 1fr;
  grid-auto-rows: 40px;
  grid-gap: 10px;
  margin: 20px;
  width: 300px;
  justify-items: stretch;
}

article:hover,
article:focus {
  justify-items: center;
}

article span {
  background-color: black;
  color: white;
  margin: 1px;
  text-align: center;
}

article,
span {
  padding: 10px;
  border-radius: 7px;
}

Result

• CSS Grid Guide: Box alignment in CSS Grid layouts
• CSS Box Alignment
• The place-items shorthand property
• The justify-self property
• The align-items property