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CSS3 Flexible Box
The Flexbox model provides for an efficient way to lay out, align, and distribute space among elements within your document - even when the viewport and the size of your elements is dynamic or unknown.
The most important idea behind the Flexbox model is that the parent container can alter its items' width/height/order to best fill the available space. A flex container expands items to fill available free space, or shrinks them to prevent overflow.1
Basic usage
Flexbox can be used to center any amount of given elements inside one element. A basic example of this is the following code:
.center-elements-inside {
display: flex;
flex-direction: row;
justify-content: center;
}
Let's break down this example. First we set the display property to "flex" so we can apply our flexbox properties. Next we declare the way flexbox will handle our elements. This can either be in a row or in a column. Setting it to row will align the elements horizontal inside the element. The column will align them vertical.
Now lets have a short look at "justify-content". This property declares how elements are distributed inside the parent element. We chose the "center" value. This means all elements inside this element will be centered. "Flex-start" will align everything to the left, and "flex-end" to the right.
There are three slightly more interesting options for justify-content you might want to try out. "space-between" will evenly space the children out across the available space, pushing the outermost children to the edges. "space-evenly" ensures the same amount of space between the items; this can look a little more centralized. "space-around" gives them equal space all around themselves, a little like a margin - two adjacent children will have double the space where they touch, and only a single amount where they're alongside the border.
justify-content defines the behavior of child elements on the main axis. What about vertical? This is where you'll need align-items, which defines how items lie on the cross-axis. Keep in mind that whether you're in a row or a column will determine what your main and your cross axis is.
flex-start, center and flex-end behave as before - left, center and right have become top, center and bottom. Other options are baseline, whereby all children will centralize themselves down a single baseline, and stretch, whereby they will stretch to fill the container.
More Information:
To get a complete understanding of Flexbox, read Understanding Flexbox Everything you need to know on the FreeCodeCamp Medium page.
For an interactive guide go through The Ultimate Guide to Flexbox — Learning Through Examples
Both of these are great resources by Ohans Emmanuel.
Yet another great visual guide that is in-depth but easy to follow can be found in A Guide to Flexbox by CSS-Tricks