align-content

This property has no effect on single line flex containers (i.e. ones with flex-wrap: nowrap).

normal

The items are packed in their default position as if no align-content value was set.

start

The items are packed flush to each other against the start edge of the alignment container in the cross axis.

center

The items are packed flush to each other in the center of the alignment container along the cross axis.

end

The items are packed flush to each other against the end edge of the alignment container in the cross axis.

flex-start

The items are packed flush to each other against the edge of the alignment container depending on the flex container's cross-start side. This only applies to flex layout items. For items that are not children of a flex container, this value is treated like start.

flex-end

The items are packed flush to each other against the edge of the alignment container depending on the flex container's cross-end side. This only applies to flex layout items. For items that are not children of a flex container, this value is treated like end.

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.

space-between

The items are evenly distributed within the alignment container along the cross axis. The spacing between each pair of adjacent items is the same. The first item is flush with the start edge of the alignment container in the cross axis, and the last item is flush with the end edge of the alignment container in the cross axis.

space-around

The items are evenly distributed within the alignment container along the cross axis. The spacing between each pair of adjacent items is the same. The empty space before the first and after the last item equals half of the space between each pair of adjacent items.

space-evenly

The items are evenly distributed within the alignment container along the cross axis. The spacing between each pair of adjacent items, the start edge and the first item, and the end edge and the last item, are all exactly the same.

stretch

If the combined size of the items along the cross axis 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 along the cross axis.

safe

Used alongside an alignment keyword. If the chosen keyword means that the item overflows the alignment container causing data loss, the item is instead aligned as if the alignment mode were start.

unsafe

Used alongside an alignment keyword. Regardless of the relative sizes of the item and alignment container and whether overflow which causes data loss might happen, the given alignment value is honored.

align-content = 
  normal                                   |
  <baseline-position>                      |
  <content-distribution>                   |
  <overflow-position>? <content-position>  

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

<content-distribution> = 
  space-between  |
  space-around   |
  space-evenly   |
  stretch        

<overflow-position> = 
  unsafe  |
  safe    

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

In this example, you can switch between three different display property values, includ flex, grid, and block. You can also switch between the different values for align-content.

HTML

<div class="wrapper">

<section>
  <div class="olive">Olive</div>
  <div class="coral">Coral</div>
  <div class="deepskyblue">Deep<br />sky<br />blue</div>
  <div class="orchid">Orchid</div>
  <div class="slateblue">Slateblue</div>
  <div class="maroon">Maroon</div>
</section>

<fieldset class="controls">
    <legend>Controls</legend>
    <div class="row">
      <label for="display">display: </label>
      <select id="display">
        <option value="block" selected>block</option>
        <option value="flex">flex</option>
        <option value="grid">grid</option>
      </select>
    </div>
    <div class="row">
      <label for="alignContent">align-content: </label>
      <select id="alignContent">
        <option value="normal" selected>normal</option>
        <option value="start">start</option>
        <option value="center">center</option>
        <option value="end">end</option>
        <option value="flex-start">flex-start</option>
        <option value="flex-end">flex-end</option>
        <option value="space-between">space-between</option>
        <option value="space-around">space-around</option>
        <option value="space-evenly">space-evenly</option>
      </select>
    </div>
    <p>CSS applied</p>
    <pre>
section {
  display: <span id="displayStyle">block</span>;
  align-content: <span id="align">normal</span>
}
    </pre>
  </fieldset>
</div>

CSS

.wrapper {
  font-size: 1.25rem;
  display: flex;
  gap: 1rem;
}
section div {
  font-family: monospace;
  padding: 3px;
}

section {
  border: solid 1.5px tomato;
  height: 300px;
  width: 300px;
  flex-wrap: wrap; /* used by flex only */
  gap: 0.2rem; /* not used by block */
}
.olive {
  background-color: olive;
}
.coral {
  background-color: coral;
}
.deepskyblue {
  background-color: deepskyblue;
}
.orchid {
  background-color: orchid;
}
.slateblue {
  background-color: slateblue;
  color: white;
}
.maroon {
  background-color: maroon;
  color: white;
}

const alignContent = document.querySelector("#alignContent");
const display = document.querySelector("#display");
const container = document.querySelector("section");
const displayStyle = document.querySelector("#displayStyle");
const alignStyle = document.querySelector("#align");
document.addEventListener("load", () => {
  updatePage();
});
alignContent.addEventListener("change", () => {
  updatePage();
});
display.addEventListener("change", () => {
  updatePage();
});
function updatePage() {
  const alVal = alignContent.value;
  const dVal = display.value;
  container.style.alignContent = alVal;
  container.style.display = dVal;
  alignStyle.innerText = alVal;
  displayStyle.innerText = dVal;
}

Result

Try changing the display value and the align-content value.

In block layout, child elements are treated as a single element, meaning space-between, space-around, and space-evenly behave differently.

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Aligning items in a flex container
• CSS Grid Guide: Box alignment in CSS Grid layouts
• CSS Box Alignment
• Block and inline layout in normal flow
• Block-level_content
• display