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CENTERING THINGSA common task for CSS is to center text or images. In fact, there are three kinds of centering:Centering lines of textCentering a block of text or an imageCentering a block or an image verticallyIn recent implementations of CSS you can also use features from level 3, which allows centering absolutely positioned elements: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: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:some random imageCENTERING VERTICALLYCSS 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.CENTERING VERTICALLY IN CSS LEVEL 3CSS 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.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 }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.When the CSS formatter supports 'flex', it's even easier:Centered!with this style sheet:div.container6 { height: 10em; display: flex; align-items: center; justify-content: center }div.container6 p { margin: 0 }
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