Web Style Sheets CSS tips & tricks
See also the index of all tips.
Centering things
A common task for CSS is to center text or images. In fact,
there are three kinds of centering:
In recent implementations of CSS you can also use features from
level 3, which allows centering absolutely positioned elements:
Centering lines of text
The most common and (therefore) easiest type of centering is
that of lines of text in a paragraph or in a heading. CSS has the
property 'text-align' for that:
P { text-align: center }
H2 { text-align: center }
renders each line in a P or in a H2 centered between its
margins, like this:
The lines in this paragraph are all
centered between the paragraph's margins, thanks to the value
'center' of the CSS property 'text-align'.
Centering a block or image
Sometimes it is not the text that needs to be centered, but the
block as a whole. Or, phrased differently: we want the left and
right margin to be equal. The way to do that is to set the margins
to 'auto'. This is normally used with a block of fixed width,
because if the block itself is flexible, it will simply take up all
the available width. Here is an example:
P.blocktext {
margin-left: auto;
margin-right: auto;
width: 8em
}
...
<P class="blocktext">This rather...
This rather narrow block of text is centered.
Note that the lines inside the block are not centered (they are
left-aligned), unlike in the earlier example.
This is also the way to center an image: make it into block of
its own and apply the margin properties to it. For example:
IMG.displayed {
display: block;
margin-left: auto;
margin-right: auto }
...
<IMG class="displayed" src="..." alt="...">
The following image is centered: 
Centering vertically
CSS level 2 doesn't have a property for centering things
vertically. There will probably be one in CSS level 3 (see below). But even in CSS2 you can center blocks vertically, by combining
a few properties. The trick is to specify that the outer block is
to be formatted as a table cell, because the contents of a table
cell can be centered vertically.
The example below centers a paragraph inside a block that has a
certain given height. A separate example shows a paragraph that is centered vertically in the browser
window, because it is inside a block that is absolutely positioned
and as tall as the window.
DIV.container {
min-height: 10em;
display: table-cell;
vertical-align: middle }
...
<DIV class="container">
<P>This small paragraph...
</DIV>
This small paragraph is vertically centered.
Centering vertically in CSS level 3
CSS level 3 offers other possibilities. At this time (2014), a
good way to center blocks vertically without using absolute
positioning (which may cause overlapping text) is still under
discussion. But if you know that overlapping text will not be a
problem in your document, you can use the 'transform' property to
center an absolutely positioned element. For example:
This paragraph is vertically centered.
For a document that looks like this:
<div class=container3>
<p>This paragraph…
</div>
the style sheet looks like this:
div.container3 {
height: 10em;
position: relative } /* 1 */
div.container3 p {
margin: 0;
position: absolute; /* 2 */
top: 50%; /* 3 */
transform: translate(0, -50%) } /* 4 */
The essential rules are:
- Make the container relatively positioned, which
declares it to be a container for absolutely positioned elements.
- Make the element itself absolutely positioned.
- Place it halfway down the container with 'top: 50%'. (Note
that 50%' here means 50% of the height of the container.)
- Use a translation to move the element up by half its own
height. (The '50%' in 'translate(0, -50%)' refers to the height of
the element itself.)
Recently (since about 2015), another technique has also become
available in several CSS implementations. It is based on the new
'flex' keyword for the 'display' property. This keyword is meant
for use in graphical user interfaces (GUIs), but nothing stops you
from using it in a document, if the document happens to have the
right structure.
This paragraph is vertically centered.
the style sheet looks like this:
div.container5 {
height: 10em;
display: flex;
align-items: center }
div.container5 p {
margin: 0 }
Centering vertically and horizontally in CSS level 3
We can extend both methods to center horizontally and
vertically at the same time.
A side-effect of making the paragraph absolutely positioned is
that it is then only as wide as it needs to be (unless we give it
an explicit width, of course). In the example below, that's
precisely what we want: We center a paragraph with just one word
(“Centered!”), so the width of the paragraph should be exactly the
width of that word.
The yellow background is there to show that the paragraph is
indeed only as wide as its contents. We assume the same mark-up as
before:
<div class=container4>
<p>Centered!
</div>
The style sheet is similar to the previous example with respect
to the vertical centering. But we now move the element halfway
across the container as well, with 'left: 50%', and at the same
time move it leftwards by half its own width in the 'translate'
transformation:
div.container4 {
height: 10em;
position: relative }
div.container4 p {
margin: 0;
background: yellow;
position: absolute;
top: 50%;
left: 50%;
margin-right: -50%;
transform: translate(-50%, -50%) }
The next example below explains why the 'margin-right: -50%' is
needed.
When the CSS formatter supports 'flex', it's even easier:
with this style sheet:
div.container6 {
height: 10em;
display: flex;
align-items: center;
justify-content: center }
div.container6 p {
margin: 0 }
i.e., the only addition is the 'justify-content: center'. Just
like 'align-items' determines the vertical alignment of the
container's contents, 'justify-content' determines the horizontal
alignment. (It's actually a bit more complex, as their names
suggest, but in a simple case that's how it works.) A side-effect
of 'flex' is that the child element, the P in this case, is
automatically made as small as possible.
Centering in the viewport in CSS level 3
The default container for absolutely positioned elements is the
viewport. (In case of a browser, that means the browser window). So
centering an element in the viewport is very simple. Here is a
complete example. (This example uses HTML5 syntax.)
<html>
<style>
body {
background: white }
section {
background: black;
color: white;
border-radius: 1em;
padding: 1em;
position: absolute;
top: 50%;
left: 50%;
margin-right: -50%;
transform: translate(-50%, -50%) }
</style>
<section>
<h1>Nicely centered</h1>
<p>This text block is vertically centered.
<p>Horizontally, too, if the window is wide enough.
</section>
You can see the result in a separate
document.
The 'margin-right: -50%' is needed to compensate the 'left:
50%'. The 'left' rule reduces the available width for the element
by 50%. The renderer will thus try to make lines that are no longer
than half the width of the container. By saying that the right
margin of the element is further to the right by that same amount,
the maximum line length is again the same as the container's width.
Try resizing the window: You'll see that each sentence is on one
line when the window is wide enough. Only when the window is too
narrow for the whole sentence will the sentence be broken over
several lines. When you remove the 'margin-right: -50%' and resize
the window again, you'll see that the sentences will be broken
already when the window is still twice as wide as the text lines.
(Using 'translate' for centering in the viewport was first
proposed by “Charlie” in an answer on Stack Overflow.)
