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
andjustify-self: flex-end
).Note that values
space-around
andspace-between
onjustify-content
property will not keep the middle item centered about the container if the adjacent items have different widths.
#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;
}
TRUE CENTER
note that the middlebox will only be truly centered if adjacent boxes are equal width
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?
Answer
Methods for Aligning Flex Items along the Main Axis
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:
- The
justify-content
keyword property, and auto
margins
justify-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 onflex-direction
and writing mode (ltr
orrtl
).space-around
~ Same asspace-between
except with half-size spaces on both ends.
Auto Margins
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.
Align group of flex items to the right, but first item to the left
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:
Place a flex item in the corner
Scenario from the question:
- placing a flex item in a corner
.box { align-self: flex-end; justify-self: flex-end; }
Center a flex item vertically and horizontally
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.
Center a flex item, and center a second flex item between the first and the edge
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.
Center a flex item when adjacent items vary in size
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
and
justify-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;
}
TRUE CENTER
The middle box will be truly centered only if adjacent boxes are equal width.
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;
}
71 short
72 centered
73 loooooooooooooooong
Here's how it works:
- The top-level div (
.container
) is a flex container. - Each child div (
.box
) is now a flex item. - Each
.box
item is givenflex: 1
in order to distribute container space equally. - Now the items are consuming all space in the row and are equal width.
- Make each item a (nested) flex container and add
justify-content: center
. - Now each
span
element is a centered flex item. - Use flex
auto
margins to shift the outerspan
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
margins
Prior 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 ofspace-between
andspace-around
. Flex items are evenly spaced (likespace-between
), except instead of half-size spaces on both ends (likespace-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)
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