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65

Consider the main axis and cross axis of a flex container:

enter image description here                                                                                                                        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 in relation to 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;
}
<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>note that the middle box will only be truly centered if adjacent boxes are equal width</p>

jsFiddle version

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:

enter image description here                                                                                                                         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?

share|improve this question
2  
How would you expect justify-self to work for a flex item? Suppose you have items a,b,c,d with extra space to distribute around them, and the flex container has justify-content: space-between, so they end up like |a b c d|. What would it mean to add e.g. justify-self: center or 'justify-self: flex-end` just to item 'b' there? Where would you expect it to go? (I see some demos in one of the answers here, but I don't see a clear way that it would work in general.) – dholbert Sep 18 '15 at 17:27
    
(Alternately: suppose we have justify-content: flex-start, so the items are crowded at the beginning like |abcd |. What would you expect it to do, if you put justify-self: [anything] on item 'b' there?) – dholbert Sep 18 '15 at 17:29
1  
@dholbert, you wrote: How would you expect justify-self to work for a flex item? I would say not so differently than auto margins already work on flex items. In your second example, justify-self: flex-end on item d would move it to the far edge. That in itself would be a great feature, which auto margins can already do. I posted an answer with a demonstration. – Michael_B Nov 22 '15 at 17:09
1  
I'm mostly seeing more use-cases here, and use-cases are great -- but the tricky bit is how to shoehorn this into the actual way that justify-content and justify-self are specified, so that cases with conflicts (like the scenarios I asked about) are clearly & sensibly defined. As I noted in my answer here, {justify|align}-selfare about aligning items within a larger box which is sized independently of the {justify|align}-self value -- and there is no such box, in the main axis, for a flex item to be aligned into. – dholbert Dec 1 '15 at 8:00
1  
RE "no different from auto margins" - I was actually asking about how you imagine justify-self and justify-content would interact, in cases where they conflict (like the scenarios I laid out). Auto margins simply do not interact with justify-content at all - they steal all of the packing space before justify-content gets a chance to use it. So, auto margins aren't really a good analog for how this would work. – dholbert Dec 1 '15 at 8:02
up vote 187 down vote
+500

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 and align-self.

The question then asks:

Why are there no justify-items and justify-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:

  1. The justify-content keyword property, and
  2. 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.

    enter image description here

  • flex-end ~ Flex items are packed toward the end of the line.

    enter image description here

  • center ~ Flex items are packed toward the center of the line.

    enter image description here

  • 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).

    enter image description here

  • space-around ~ Same as space-between except with half-size spaces on both ends.

    enter image description here

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.

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.

enter image description here

enter image description here

Other useful scenarios:

enter image description here

enter image description here

enter image description here

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; }

enter image description here

Center a flex item vertically and horizontally

enter image description here

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).

enter image description here

enter image description here

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: Absolute positioning in flexbox

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 and space-between on justify-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).

For now, however, the flexbox spec allows for absolute positioning of flex items. This is not an ideal solution since, like all absolutely positioned elements, the item is removed from the document flow. But it does perfectly center the middle item and may be useful in some cases.

enter image description here

For another solution, not involving absolute positioning, see:

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.

enter image description here

(credit: @oriol for the code, and @crl for the label)

PLAYGROUND (includes code for all examples above)

share|improve this answer
3  
Absolutely amazing. Didn't know about the margin: auto concept in Flexbox until now. – Sam Pakvis Aug 31 at 12:07
up vote 34 down vote

I know not an answer but I like to contribute to this matter for what it's worth. It would be great if they could release justify-self for flexbox to make it truly flexible.

In my believe when there are multiple items on the axis, the most logical way for justify-self to behave is to align itself to it's nearest neighbors (or edge) as demonstrated below.

I truly hope, W3C watches this and will at least consider it. =)

enter image description here

This way you can have an item that is truly centered regardless of the size of the left and right box. When one of the boxes reaches the point of the center box it will simply push it until there is no more space to distribute.

enter image description here

The ease of making awesome layouts are endless, take a look at this "complex" example.

enter image description here

share|improve this answer
3  
@Mark this is amazing! Thank you for taking the time to draft this. I hope you don’t mind but I’ve submitted it here: discourse.wicg.io/t/flexbox-justify-self-property/1146 – Zaqx Oct 15 '15 at 2:52
    
@Zaqx sure, the more exposure the better. Seriously hope they will find a way to make this reality, would love to get my hands dirty on this. – Mark Oct 18 '15 at 19:52
    
Hi Mark, @Zaqx, I posted an answer that may cover some of the use-cases mentioned in this answer. I've discovered that a lot can still be done in the main axis without needing justify-self and justify-items. – Michael_B Nov 23 '15 at 5:30
1  
@Michael_B Glad you're learning more about flexbox and appreciate that you took the time to format your notes into an answer. Margin: auto is indeed cool (though you'll find the browsers that implemented the 2009 and 2012 versions of the flexbox spec have no equivalent feature which drastically reduces the ability to use it on a production site). However, even with browsers that support margin:auto, the main use case here of an item that is centered relative to its container while space exists is still impossible without a justify-self property or similar. – Zaqx Nov 23 '15 at 9:34
    
@Zaqx, thanks for the feedback. And yes, I agree with you, auto margins are not a complete substitute for justify-self . I was careful to word my previous comment by saying auto margins will cover some of the use-cases. As you noted, it can't center an item in place when adjacent items are of different sizes. However, it does provide more options for aligning flex items. When I first started learning about flexbox I thought justify-content and invisible spacer items were the only way. – Michael_B Nov 24 '15 at 14:40
up vote 5 down vote

This was asked on the www-style list, and Tab Atkins (spec editor) provided an answer explaining why. I'll elaborate on that a bit here.

To start out, let's initially assume our flex container is single-line (flex-wrap: nowrap). In this case, there's clearly an alignment difference between the main axis and the cross axis -- there are multiple items stacked in the main axis, but only one item stacked in the cross axis. So it makes sense to have a customizeable-per-item "align-self" in the cross axis (since each item is aligned separately, on its own), whereas it doesn't make sense in the main axis (since there, the items are aligned collectively).

For multi-line flexbox, the same logic applies to each "flex line". In a given line, items are aligned individually in the cross axis (since there's only one item per line, in the cross axis), vs. collectively in the main axis.

Here's another way of phrasing it: so, all of the *-self and *-content properties are about how to distribute extra space around things. But the key difference is that the *-self versions are for cases where there's only a single thing in that axis, and the *-content versions are for when there are potentially many things in that axis. The one-thing vs. many-things scenarios are different types of problems, and so they have different types of options available -- for example, the space-around / space-between values make sense for *-content, but not for *-self.

SO: In a flexbox's main axis, there are many things to distribute space around. So a *-content property makes sense there, but not a *-self property.

In contrast, in the cross axis, we have both a *-self and a *-content property. One determines how we'll distribute space around the many flex lines (align-content), and one determines how to distribute space around individual flex items in the cross axis, within a given flex line.

(I'm ignoring *-items properties here, since they simply establish defaults for *-self.)

share|improve this answer
1  
I understand how a justify-self could be a problem on an axis with multiple items as opposed to one item. However I really hope they will look into it further, in a multiple items situation how would justify-self behave, because there are some situations where this could be really useful. For example (stackoverflow.com/questions/32378953/…). – Mark Sep 15 '15 at 9:36
    
Nice find on that mailing list. Trying to wrap my head around it, as I still see good use case possibilities for justify-self. – Michael_B Sep 15 '15 at 10:26
    
I added a bit more clarification, explaining how *-self is about cases where we only have one thing to align whereas *-content is about cases where we have many things to align. Does that make it a bit clearer? – dholbert Sep 15 '15 at 14:11
    
Yes, it's more clear. I appreciate the explanation. But it seems to all boil down to a simple preference by the spec authors, not a particular need based on logic. From the mailing list you referenced: The *-self properties only work if the child is all alone in that axis. Why? There doesn't seem to be a need for this. It's an arbitrary decision. A preference. In fact, auto margins, also part of the spec, enable *-self alignment with multiple items in the line. No clear reason why keyword properties can't, as well. – Michael_B Nov 22 '15 at 16:30

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