Consider the main axis and cross axis of a flex container:
Source: W3C
To align flex items along the main axis there is one property:
To align flex items along the cross axis there are three properties:
In the image above, the main axis is horizontal and the cross axis is vertical. These are the default directions of a flex container.
However, these directions can be easily interchanged with the flex-direction
property.
/* main axis is horizontal, cross axis is vertical */
flex-direction: row;
flex-direction: row-reverse;
/* main axis is vertical, cross axis is horizontal */
flex-direction: column;
flex-direction: column-reverse;
(The cross axis is always perpendicular to the main axis.)
My point in describing how the axes' work is that there doesn't seem to be anything special about either direction. Main axis, cross axis, they're both equal in terms of importance and flex-direction
makes it easy to switch back and forth.
So why does the cross axis get two additional alignment properties?
Why are align-content
and align-items
consolidated into one property for the main axis?
Why does the main axis not get a justify-self
property?
Scenarios where these properties would be useful:
placing a flex item in the corner of the flex container
#box3 { align-self: flex-end; justify-self: flex-end; }
making a group of flex items align-right (justify-content: flex-end
) but have the first item align left (justify-self: flex-start
)
Consider a header section with a group of nav items and a logo. With justify-self
the logo could be aligned left while the nav items stay far right, and the whole thing adjusts smoothly ("flexes") to different screen sizes.
in a row of three flex items, affix the middle item to the center of the container (justify-content: center
) and align the adjacent items to the container edges (justify-self: flex-start
and justify-self: flex-end
).
Note that values space-around
and space-between
on
justify-content
property will not keep the middle item centered about the container if the adjacent items have different widths.
As of this writing, there is no mention of justify-self
or justify-items
in the flexbox spec.
However, in the CSS Box Alignment Module, which is the W3C's unfinished proposal to establish a common set of alignment properties for use across all box models, there is this:
Source: W3C
You'll notice that justify-self
and justify-items
are being considered... but not for flexbox.
I'll end by reiterating the main question:
Why are there no "justify-items" and "justify-self" properties?
As stated in the question:
To align flex items along the main axis there is one property:
justify-content
To align flex items along the cross axis there are three properties:
align-content
,align-items
andalign-self
.
The question then asks:
Why are there no
justify-items
andjustify-self
properties?
One answer may be: Because they're not necessary.
The flexbox specification provides two methods for aligning flex items along the main axis:
justify-content
keyword property, andauto
marginsjustify-content
The justify-content
property aligns flex items along the main axis of the flex container.
It is applied to the flex container but only affects flex items.
There are five alignment options:
flex-start
~ Flex items are packed toward the start of the line.
flex-end
~ Flex items are packed toward the end of the line.
center
~ Flex items are packed toward the center of the line.
space-between
~ Flex items are evenly spaced, with the first item aligned to one edge of the container and the last item aligned to the opposite edge. The edges used by the first and last items depends on flex-direction
and writing mode (ltr
or rtl
).
space-around
~ Same as space-between
except with half-size spaces on both ends.
With auto
margins, flex items can be centered, spaced away or packed into sub-groups.
Unlike justify-content
, which is applied to the flex container, auto
margins go on flex items.
They work by consuming all free space in the specified direction.
Scenario from the question:
making a group of flex items align-right (
justify-content: flex-end
) but have the first item align left (justify-self: flex-start
)Consider a header section with a group of nav items and a logo. With
justify-self
the logo could be aligned left while the nav items stay far right, and the whole thing adjusts smoothly ("flexes") to different screen sizes.
Other useful scenarios:
Scenario from the question:
- placing a flex item in a corner
.box { align-self: flex-end; justify-self: flex-end; }
margin: auto
is an alternative to justify-content: center
and align-items: center
.
Instead of this code on the flex container:
.container {
justify-content: center;
align-items: center;
}
You can use this on the flex item:
.box56 {
margin: auto;
}
This alternative is useful when centering a flex item that overflows the container.
A flex container aligns flex items by distributing free space.
Hence, in order to create equal balance, so that a middle item can be centered in the container with a single item alongside, a counterbalance must be introduced.
In the examples below, invisible third flex items (boxes 61 & 68) are introduced to balance out the "real" items (box 63 & 66).
Of course, this method is nothing great in terms of semantics.
Alternatively, you can use a pseudo-element instead of an actual DOM element. Or you can use absolute positioning. All three methods are covered here: Center and bottom-align flex items
NOTE: The examples above will only work – in terms of true centering – when the outermost items are equal height/width. When flex items are different lengths, see next example.
Scenario from the question:
in a row of three flex items, affix the middle item to the center of the container (
justify-content: center
) and align the adjacent items to the container edges (justify-self: flex-start
andjustify-self: flex-end
).Note that values
space-around
andspace-between
onjustify-content
property will not keep the middle item centered in relation to the container if the adjacent items have different widths (see demo).
As noted, unless all flex items are of equal width or height (depending on flex-direction
), the middle item cannot be truly centered. This problem makes a strong case for a justify-self
property (designed to handle the task, of course).
#container {
display: flex;
justify-content: space-between;
background-color: lightyellow;
}
.box {
height: 50px;
width: 75px;
background-color: springgreen;
}
.box1 {
width: 100px;
}
.box3 {
width: 200px;
}
#center {
text-align: center;
margin-bottom: 5px;
}
#center > span {
background-color: aqua;
padding: 2px;
}
<div id="center">
<span>TRUE CENTER</span>
</div>
<div id="container">
<div class="box box1"></div>
<div class="box box2"></div>
<div class="box box3"></div>
</div>
<p>The middle box will be truly centered only if adjacent boxes are equal width.</p>
Here are two methods for solving this problem:
Solution #1: Absolute Positioning
The flexbox spec allows for absolute positioning of flex items. This allows for the middle item to be perfectly centered regardless of the size of its siblings.
Just keep in mind that, like all absolutely positioned elements, the items are removed from the document flow. This means they don't take up space in the container and can overlap their siblings.
In the examples below, the middle item is centered with absolute positioning and the outer items remain in-flow. But the same layout can be achieved in reverse fashion: Center the middle item with justify-content: center
and absolutely position the outer items.
Solution #2: Nested Flex Containers (no absolute positioning)
.container {
display: flex;
}
.box {
flex: 1;
display: flex;
justify-content: center;
}
.box71 > span { margin-right: auto; }
.box73 > span { margin-left: auto; }
/* non-essential */
.box {
align-items: center;
border: 1px solid #ccc;
background-color: lightgreen;
height: 40px;
}
<div class="container">
<div class="box box71"><span>71 short</span></div>
<div class="box box72"><span>72 centered</span></div>
<div class="box box73"><span>73 loooooooooooooooong</span></div>
</div>
Here's how it works:
.container
) is a flex container..box
) is now a flex item..box
item is given flex: 1
in order to distribute container space equally.justify-content: center
.span
element is a centered flex item.auto
margins to shift the outer span
s left and right.You could also forgo justify-content
and use auto
margins exclusively.
But justify-content
can work here because auto
margins always have priority. From the spec:
8.1. Aligning with
auto
marginsPrior to alignment via
justify-content
andalign-self
, any positive free space is distributed to auto margins in that dimension.
justify-content: space-same (concept)
Going back to justify-content
for a minute, here's an idea for one more option.
space-same
~ A hybrid of space-between
and space-around
. Flex items are evenly spaced (like space-between
), except instead of half-size spaces on both ends (like space-around
), there are full-size spaces on both ends.This layout can be achieved with ::before
and ::after
pseudo-elements on the flex container.
(credit: @oriol for the code, and @crl for the label)
UPDATE: Browsers have begun implementing space-evenly
, which accomplishes the above. See this post for details: Equal space between flex items
PLAYGROUND (includes code for all examples above)