The Flexbox Layout (Flexible Box) module (currently a W3C Candidate Recommendation) aims at providing a more efficient way to lay out, align and distribute space among items in a container, even when their size is unknown and/or dynamic (thus the word "flex").
The main idea behind the flex layout is to give the container the ability to alter its items' width/height (and order) to best fill the available space (mostly to accomodate to all kind of display devices and screen sizes). A flex container expands items to fill available free space, or shrinks them to prevent overflow.
Most importantly, the flexbox layout is direction-agnostic as opposed to the regular layouts (block which is vertically-based and inline which is horizontally-based). While those work well for pages, they lack flexibility (no pun intended) to support large or complex applications (especially when it comes to orientation changing, resizing, stretching, shrinking, etc.).
Note: Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the Grid layout is intended for larger scale layouts.
Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Some of them are meant to be set on the container (parent element, known as "flex container") whereas the others are meant to be set on the children (said "flex items").
If regular layout is based on both block and inline flow directions, the flex layout is based on "flex-flow directions". Please have a look at this figure from the specification, explaining the main idea behind the flex layout.
Basically, items will be layed out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross- end).
- main axis - The main axis of a flex container is the primary axis along
which flex items are laid out. Beware, it is not necessarily horizontal; it
depends on the
justify-content
property (see below). - main-start | main-end - The flex items are placed within the container starting from main-start and going to main-end.
- main size - A flex item's width or height, whichever is in the main dimension, is the item's main size. The flex item's main size property is either the ‘width’ or ‘height’ property, whichever is in the main dimension.
- cross axis - The axis perpendicular to the main axis is called the cross axis. Its direction depends on the main axis direction.
- cross-start | cross-end - Flex lines are filled with items and placed into the container starting on the cross-start side of the flex container and going toward the cross-end side.
- cross size - The width or height of a flex item, whichever is in the cross dimension, is the item's cross size. The cross size property is whichever of ‘width’ or ‘height’ that is in the cross dimension.
Applies to: parent flex container element
This defines a flex container; inline or block depending on the given value. Thus, it enables a flex context for all its direct children.
display: other values | flex | inline-flex;
Please note that:
- CSS columns have no effect on a flex container
float
,clear
andvertical-align
have no effect on a flex item
Applies to: parent flex container element
This establishes the main-axis, thus defining the direction flex items are placed in the flex container.
flex-direction: row | row-reverse | column | column-reverse
row
(default): left to right inltr
; right to left inrtl
row-reverse
: right to left inltr
; left to right inrtl
column
: same asrow
but top to bottomcolumn-reverse
: same asrow-reverse
but top to bottom
Applies to: parent flex container element
This defines whether the flex container is single-line or multi-line, and the direction of the cross-axis, which determines the direction new lines are stacked in.
flex-wrap: nowrap | wrap | wrap-reverse
nowrap
(default): single-line / left to right inltr
; right to left inrtl
wrap
: multi-line / left to right inltr
; right to left inrtl
wrap-reverse
: multi-line / right to left inltr
; left to right inrtl
Applies to: parent flex container element
This is a shorthand flex-direction
and flex-wrap
properties, which
together define the flex container's main and cross axes. Default is row nowrap
;
flex-flow: <'flex-direction'> || <'flex-wrap'>
Applies to: parent flex container element
This defines the alignment along the main axis. It helps distribute extra free space leftover when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.
justify-content: flex-start | flex-end | center | space-between | space-around
flex-start
(default): items are packed toward the start lineflex-end
: items are packed toward to end linecenter
: items are centered along the linespace-between
: items are evenly distributed in the line; first item is on the start line, last item on the end linespace-around
: items are evenly distributed in the line with equal space around them
Applies to: parent flex container element
This defines the default behaviour for how flex items are laid out along the
cross axis on the current line. Think of it as the justify-content
version
for the cross-axis (perpendicular to the main-axis).
align-items: flex-start | flex-end | center | baseline | stretch
flex-start
: cross-start margin edge of the items is placed on the cross-start lineflex-end
: cross-end margin edge of the items is placed on the cross-end linecenter
: items are centered in the cross-axisbaseline
: items are aligned such as their baselines alignstretch
(default): stretch to fill the container (still respect min-width/max-width)
Applies to: parent flex container element
This aligns a flex container's lines within when there is extra space in the
cross-axis, similar to how justify-content
aligns individual items within
the main-axis.
Note: this property has no effect when the flexbox has only a single line.
align-content: flex-start | flex-end | center | space-between | space-around | stretch
flex-start
: lines packed to the start of the containerflex-end
: lines packed to the end of the containercenter
: lines packed to the center of the containerspace-between
: lines evenly distributed; the first line is at the start of the container while the last one is at the endspace-around
: lines evenly distributed with equal space between themstretch
(default): lines stretch to take up the remaining space
Applies to: child element / flex item
By default, flex items are layed out in the source order. However, the order
property controls the order in which they appear in their container.
order: <integer>
Applies to: child element / flex item
This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.
If all items have flex-grow
set to 1, every child will set to an equal size
inside the container. If you were to give one of the children a value of 2,
that child would take up twice as much space as the others.
flex-grow: <number> (default 0)
Negative numbers are invalid.
Applies to: child element / flex item
This defines the ability for a flex item to shrink if necessary.
flex-shrink: <number> (default 1)
Negative numbers are invalid.
Applies to: child element / flex item
This defines the default size of an element before the remaining space is distributed.
flex-basis: <length> | auto (default auto)
Applies to: child element / flex item
This is the shorthand for flex-grow
, flex-shrink
and flex-basis
. The
second and third parameters (flex-shrink
, flex-basis
) are optional.
Default is 0 1 auto
.
flex: none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]
Applies to: child element / flex item
This allows the default alignment or the one specified by align-items
to be overridden for individual flex items.
Please see the align-items
explanation to understand the available values.
align-self: auto | flex-start | flex-end | center | baseline | stretch
Let's start with a very very simple example, solving an almost daily problem: perfect centering. It couldn't be any simpler if you use flexbox.
.parent {
display: flex;
height: 300px; /* Or whatever */
}
.child {
width: 100px; /* Or whatever */
height: 100px; /* Or whatever */
margin: auto; /* Magic! */
}
This relies on the fact a margin set to auto
in a flex container absorb
extra space. So setting a vertical margin of auto
will make the item
perfectly centered in both axis.
Now let's use some more properties. Consider a list of 6 items, all with a fixed dimensions in a matter of aesthetics but they could be auto-sized. We want them to be evenly and nicely distributed on the horizontal axis so that when we resize the browser, everything is fine (without media queries!).
.flex-container {
/* We first create a flex layout context */
display: flex;
/* Then we define the flow direction and if we allow the items to wrap
* Remember this is the same as:
* flex-direction: row;
* flex-wrap: wrap;
*/
flex-flow: row wrap;
/* Then we define how is distributed the remaining space */
justify-content: space-around;
}
Done. Everything else is just some styling concern. Below is a pen featuring this example. Be sure to go to CodePen and try resizing your windows to see what happen.
Check out this Pen!
<script async src="http://codepen.io/assets/embed/ei.js"></script>
Let's try something else. Imagine we have a right-aligned navigation on the very top of our website, but we want it to be centered on medium-sized screens and single-columned on small devices. Easy enough.
/* Large */
.navigation {
display: flex;
flex-flow: row wrap;
/* This aligns items to the end line on main-axis */
justify-content: flex-end;
}
/* Medium screens */
@media all and (max-width: 800px) {
.navigation {
/* When on medium sized screens, we center it by evenly distributing
empty space around items */
justify-content: space-around;
}
}
/* Small screens */
@media all and (max-width: 500px) {
.navigation {
/* On small screens, we are no longer using row direction but column */
flex-direction: column;
}
}
Check out this Pen!
<script async src="http://codepen.io/assets/embed/ei.js"></script>
Let's try something even better by playing with flex items flexibility! What about a mobile-first 3-columns layout with full-width header and footer. And independent from source order.
.wrapper {
display: flex;
flex-flow: row wrap;
}
/* We tell all items to be 100% width */
.header, .main, .nav, .aside, .footer {
flex: 1 100%;
}
/* We rely on source order for mobile-first approach
* in this case:
* 1. header
* 2. nav
* 3. main
* 4. aside
* 5. footer
*/
/* Medium screens */
@media all and (min-width: 600px) {
/* We tell both sidebars to share a row */
.aside { flex: 1 auto; }
}
/* Large screens */
@media all and (min-width: 800px) {
/* We invert order of first sidebar and main
* And tell the main element to take twice as much width as the other two sidebars
*/
.main { flex: 2 0px; }
.aside-1 { order: 1; }
.main { order: 2; }
.aside-2 { order: 3; }
.footer { order: 4; }
}
Check out this Pen!
<script async src="http://codepen.io/assets/embed/ei.js"></script>
- Flexbox in the CSS specifications
- Flexbox at MDN
- Flexbox at Opera
- Diving into Flexbox by Bocoup
- Mixing syntaxes for best browser support on CSS-Tricks
- Flexbox by Raphael Goetter (FR)
- Flexplorer by Bennett Feely
- (modern) means the recent syntax from the specification (e.g.
display: flex;
) - (hybrid) means an odd unofficial syntax from 2011 (e.g.
display: flexbox;
) - (old) means the old syntax from 2009 (e.g.
display: box;
)
Chrome | Safari | Firefox | Opera | IE | Android | iOS |
---|---|---|---|---|---|---|
21+ (modern) 20- (old) |
3.1+ (old) | 2-21 (old) 22+ (new) |
12.1+ (modern) | 10+ (hybrid) | 2.1+ (old) | 3.2+ (old) |
Blackberry browser 10+ supports the new syntax.
For more informations about how to mix syntaxes in order to get the best browser support, please refer to this article (CSS-Tricks) or this article (DevOpera).
A Sass @mixin to help ease the pain:
@mixin flexbox() {
display: -webkit-box;
display: -moz-box;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
}
@mixin flex($values) {
-webkit-box-flex: $values;
-moz-box-flex: $values;
-webkit-flex: $values;
-ms-flex: $values;
flex: $values;
}
@mixin order($val) {
-webkit-box-ordinal-group: $val;
-moz-box-ordinal-group: $val;
-ms-flex-order: $val;
-webkit-order: $val;
order: $val;
}
.wrapper {
@include flexbox();
}
.item {
@include flex(1 200px);
@include order(2);
}