CSS Flexible Box Layout

CSS Flexible Box Layout, commonly known as Flexbox, is a CSS 3 web layout model. It is in the W3C's candidate recommendation (CR) stage. The flex layout allows responsive elements within a container to be automatically arranged depending upon screen size (or device).

Concepts

Most web pages are written in a combination of HTML (Hypertext Markup Language) and CSS (Cascading Style Sheets). In short, HTML specifies the content and logical structure of the page, while the CSS specifies how it looks: its colors, fonts, formatting, layout, and styling.

CSS flex-box layout is a particular way to specify the layout of HTML pages.

One of the most defining features of the flex layout is its ability to form-fit, based on its viewing environment. Flex boxes can adjust in size—either decreasing, to avoid unnecessarily monopolizing space, or increasing to make room for contents to be constrained within its boundaries. Moreover, the flex layout is less restrictive in terms of content flow than those, for example, of the block and inline display types, which are generally uni-directional. Indeed, not only can flex directional flow be specified, at the style level, as rightwards, leftwards, upwards, or downwards; individual items within a flex container may also be automatically reordered and rearranged to suit the available layout space.

History

In the 2000s the intensive use of the Web by mobile agents motivated "liquid layouts" and responsive elements for the growing variety of screen sizes. In the 2010s, the intensive use of popular JavaScript layout frameworks, such as Bootstrap, inspired CSS flex-box and grid layout specifications.

CSS 3 modules included solutions akin to this, like flexbox and grid.

Terminology

Following are a few terms associated with the flex layout model

Flex container

Parent element that holds all flex items. Using the CSS display property, the container can be defined as either flex or inline-flex.

Flex item

Any direct child element held within the flex container is considered a flex item. Any text within the container element is wrapped in an unknown flex item.

Axes

Each flex box contains two axes: the main and cross axes. The main axis is the axis on which the items align with each other. The cross axis is perpendicular to the main axis.

Flex-direction

Establishes main axis. Possible arguments: row (default), row-reverse, column, column-reverse.

Justify-content

Determines how content gets placed on the main axis on the current line. Optional arguments: left, right, center, space-between, space-around.

Align-items

Determines the default for how flex items get placed on the cross axis on each line.

Align-content

Determines the default for how cross axis lines are aligned.

Align-self

Determines how a single item is placed along the cross axis. This overrides any defaults set by align-items.

Directions

The main-start/main-end sides determine where to start placing flex items within the flex container, starting from the main-start end and going to the main-end end. The cross-start/cross-end sides determine where flex lines get filled with flex items from cross-start to cross-end.

Order

Places elements in groups and determines which order they are to be placed in within the container.

Flex-flow

Shorthands flex-direction and flex-wrap to place the flex content.

Lines

Flex items can either be placed on a singular line or on multiple lines as defined by the flex-wrap property, which controls both the direction of the cross axis and how lines stack within the container.

Dimensions

Main size and cross size are essentially the height and width of the flex container, each dealing with the main and cross axes respectively.

Designate a flex box

Designating an element as a flex element is relatively easy. All that is necessary is to set the display property to either flex or inline-flex as follows:

display: flex;

Or:

display: inline-flex;

By setting the display to one of the two values above, an element becomes a flex container and its children flex items. Setting the display to flex makes the container a block-level element, while setting the display to inline-flex makes the container an inline-level element.

Align to center

One of flexbox's advantages is the ability to easily align items within the container to the center of a page, both vertically and horizontally.

display: flex;
align-items: center;
justify-content: center;