CSS transforms are based on the size of the elements so if you don't know how tall or wide your element is, you can position it absolutely 50% from the top and left of a relative container and translate it by 50% left and upwards to center it vertically and horizontally.

Keep in mind that with this technique, the element could end being rendered at a non-integer pixel boundary, making it look blurry. See this answer in SO for a workaround.

HTML

<div class="container">
  <div class="element"></div>
</div>

CSS

.container {
  position: relative;
}

.element {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

View example in JSFiddle

CROSS BROWSER COMPATIBILITY

The transform property needs prefixes to be supported by older browsers. Prefixes are needed for Chrome<=35, Safari<=8, Opera<=22, Android Browser<=4.4.4, and IE9. CSS transforms are not supported by IE8 and older versions.

Here is a common transform declaration for the previous example:

-webkit-transform: translate(-50%, -50%); /* Chrome, Safari, Opera, Android */
    -ms-transform: translate(-50%, -50%); /* IE 9 */
        transform: translate(-50%, -50%);

For more information see canIuse.

MORE INFORMATION

• The element is being positioned according to the first non-static parent (position: relative, absolute, or fixed). Explore more in this fiddle and this documentation topic.

• For horizontal-only centering, use left: 50% and transform: translateX(-50%). The same goes for vertical-only centering: center with top: 50% and transform: translateY(-50%).

• Using a non-static width/height elements with this method of centering can cause the centered element to appear squished. This mostly happens with elements containing text, and can be fixed by adding: margin-right: -50%; and margin-bottom: -50%;. View this fiddle for more information.

HTML:

<div class="container">
  <img src="http://lorempixel.com/400/200" />
</div>

CSS:

html, body, .container {
  height: 100%;
}    
.container {
  display: flex;
  justify-content: center; /* horizontal center */
}
img {
  align-self: center; /* vertical center */
}

View Result

---

HTML:

<img src="http://lorempixel.com/400/200" />

CSS:

html, body {
  height: 100%;
}   
body {
  display: flex;
  justify-content: center; /* horizontal center */
  align-items: center;     /* vertical center */
}

View Result

See Dynamic Vertical and Horizontal Centering under the Flexbox documentation for more details on flexbox and what the styles mean.

Browser Support

Flexbox is supported by all major browsers, except IE versions before 10.

Some recent browser versions, such as Safari 8 and IE10, require vendor prefixes.

For a quick way to generate prefixes there is Autoprefixer, a third-party tool.

For older browsers (like IE 8 & 9) a Polyfill is available.

For a more detailed look at flexbox browser support, see this answer.

Working in old browsers (IE >= 8)

Automatic margins, paired with values of zero for the left and right or top and bottom offsets, will center an absolutely positioned elements within its parent.

View Result

HTML

<div class="parent">
  <img class="center" src="http://lorempixel.com/400/200/" />
</div>

CSS

.parent {
  position: relative;
  height: 500px;
}

.center {
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

Elements that don't have their own implicit width and height like images do, will need those values defined.

Other resources: Absolute Centering in CSS

This technique works even when the container's dimensions are unknown.

Set up a "ghost" element inside the container to be centered that is 100% height, then use vertical-align: middle on both that and the element to be centered.

CSS

/* This parent can be any width and height */
.block {
  text-align: center;

  /* May want to do this if there is risk the container may be narrower than the element inside */
  white-space: nowrap;
}
 
/* The ghost element */
.block:before {
  content: '';
  display: inline-block;
  height: 100%;
  vertical-align: middle;

  /* There is a gap between ghost element and .centered,
  caused by space character rendered. Could be eliminated by
  nudging .centered (nudge distance depends on font family),
  or by zeroing font-size in .parent and resetting it back
  (probably to 1rem) in .centered. */
  margin-right: -0.25em;
}

/* The element to be centered, can also be of any width and height */ 
.centered {
  display: inline-block;
  vertical-align: middle;
  width: 300px;
  white-space: normal; /* Resetting inherited nowrap behavior */
}

HTML

<div class="block">
  <div class="centered"></div>
</div>

The most common and easiest type of centering is that of lines of text in an element. CSS has the rule text-align: center for this purpose:

HTML

<p>Lorem ipsum</p>

CSS

p {
    text-align: center;
}

This does not work for centering entire block elements. text-align controls only alignment of inline content like text in its parent block element.

See more about text-align in Typography section.

We will see how to center content based on the height of a near element.

Compatibility: IE8+, all other modern browsers.

HTML

<div class="content">
  <div class="position-container">
    <div class="thumb">
      <img src="http://lorempixel.com/400/200/">
    </div>
    <div class="details">
      <p class="banner-title">text 1</p>
      <p class="banner-text">content content content content content content content content content content content content content content</p>
      <button class="btn">button</button>
    </div>
  </div>
</div>

CSS

.content * {
  box-sizing: border-box;
}
.content .position-container {
  display: table;
}
.content .details {
  display: table-cell;
  vertical-align: middle;
  width: 33.333333%;
  padding: 30px;
  font-size: 17px;
  text-align: center;
}
.content .thumb {
  width: 100%;
}
.content .thumb img {
  width: 100%;
}

Link to JSFiddle

The main points are the 3 .thumb, .details and .position-container containers:

• The .position-container must have display: table.

• The .details must have the real width set width: .... and display: table-cell, vertical-align: middle.

• The .thumb must have width: 100% if you want that it will take all the remaining space and it will be influenced by the .details width.

• The image (if you have an image) inside .thumb should have width: 100%, but it is not necessary if you have correct proportions.

Supported by IE11+

View Result

Use these 3 lines to vertical align practically everything. Just make sure the div/image you apply the code to has a parent with a height.

CSS

div.vertical {
  position: relative;
  top: 50%;
  transform: translateY(-50%);
}

HTML

<div class="vertical">Vertical aligned text!</div>

HTML

<div class="wrap">
    <img src="http://lorempixel.com/400/200/" />
</div>

CSS

.wrap {
    height: 50px;/* max image height */
    width: 100px;
    border: 1px solid blue;
    text-align: center;
}
.wrap:before {
  content:"";
  display: inline-block;
  height: 100%;
  vertical-align: middle;
  width: 1px;
}

img {
    vertical-align: middle;
}

One could easily center a child element using table display property.

HTML

<div class="wrapper">
    <div class="parent">
        <div class="child"></div>
    </div>
</div>

CSS

.wrapper { 
    display: table; 
    vertical-align: center; 
    width: 200px; 
    height: 200px; 
    background-color: #9e9e9e; 
}
.parent { 
    display: table-cell; 
    vertical-align: middle; 
    text-align: center; 
}
.child { 
    display: inline-block; 
    vertical-align: middle; 
    text-align: center; 
    width: 100px; 
    height: 100px; 
    background-color: teal; 
}

The calc() function is the part of a new syntax in CSS3 in which you can calculate (mathematically) what size/position your element occupies by using a variety of values like pixels, percentages, etc. Note:- Whenever you use this function, always take care of the space between two values calc(100% - 80px).

CSS

.center {
    position: absolute;
    height: 50px;
    width: 50px;
    background: red;
    top: calc(50% - 50px / 2); /* height divided by 2*/
    left: calc(50% - 50px / 2); /* width divided by 2*/
}

HTML

<div class="center"></div>

Applying css intuitively doesn't produce the desired results because

• vertical-align:middle isn't applicable to block-level elements
• margin-top:auto and margin-bottom:auto used values would compute as zero
• margin-top:-50% percentage-based margin values are calculated relative to the width of containing block

For widest browser support, a workaround with helper elements:

HTML

<div class="vcenter--container">
  <div class="vcenter--helper">
    <div class="vcenter--content">
      <!--stuff-->
    </div>
  </div>
</div>

CSS

.vcenter--container {
  display: table;
  height: 100%;
  position: absolute;
  overflow: hidden;
  width: 100%;
}
.vcenter--helper {
  display: table-cell;
  vertical-align: middle;
}
.vcenter--content {
  margin: 0 auto;
  width: 200px;
}

jsfiddle from original question. This approach

• works with dynamic height elements
• respects content flow
• is supported by legacy browsers

You can also use line-height to center vertically a single line of text inside a container :

CSS

div {
    height: 200px;
    line-height: 200px;
}

That's quite ugly, but can be useful inside an <input /> element. The line-height property works only when the text to be centered spans a single line. If the text wraps into multiple lines, the resulting output won't be centered.

The following technique allows you to add your content to an HTML element and center it both horizontally and vertically without worrying about its height or width.

The outer container

• should have display: table;

The inner container

• should have display: table-cell;
• should have vertical-align: middle;
• should have text-align: center;

The content box

• should have display: inline-block;
• should re-adjust the horizontal text-alignment to eg. text-align: left; or text-align: right;, unless you want text to be centered

Demo

HTML

<div class="outer-container">
   <div class="inner-container">
     <div class="centered-content">
        You can put anything here!
     </div>
   </div>
</div>

CSS

body {
    margin : 0;
}

.outer-container {
    position : absolute;
    display: table;
    width: 100%; /* This could be ANY width */
    height: 100%; /* This could be ANY height */
    background: #ccc;
}

.inner-container {
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}

.centered-content {
    display: inline-block;
    text-align: left;
    background: #fff;
    padding: 20px;
    border: 1px solid #000;
}

See also this Fiddle!

If the size of your content is fixed, you can use absolute positioning to 50% with margin that reduces half of your content's width and height:

HTML

<div class="center">
    Center vertically and horizontally
</div>

CSS

.center {
    position: absolute;
    background: #ccc;

    left: 50%;
    width: 150px;
    margin-left: -75px;  /* width * -0.5 */

    top: 50%;
    height: 200px;
    margin-top: -100px;  /* height * -0.5 */
}

---

Horizontal centering with only fixed width

You can center the element horizontally even if you don't know the height of the content:

HTML

<div class="center">
    Center only horizontally
</div>

CSS

.center {
    position: absolute;
    background: #ccc;

    left: 50%;
    width: 150px;
    margin-left: -75px;  /* width * -0.5 */
}

---

Vertical centering with fixed height

You can center the element vertically if you know the element's height:

HTML

<div class="center">
    Center only vertically
</div>

CSS

.center {
    position: absolute;
    background: #ccc;

    top: 50%;
    height: 200px;
    margin-top: -100px;  /* width * -0.5 */
}

Objects can be centered by using margin: 0 auto; if they are block elements and have a defined width.

HTML

<div class="containerDiv">
    <div id="centeredDiv"></div>
</div>

<div class="containerDiv">
    <p id="centeredParagraph">This is a centered paragraph.</p>
</div>

<div class="containerDiv">
    <img id="centeredImage" src="https://i.kinja-img.com/gawker-media/image/upload/s--c7Q9b4Eh--/c_scale,fl_progressive,q_80,w_800/qqyvc3bkpyl3mfhr8all.jpg" />
</div>

CSS

.containerDiv {
    width: 100%;
    height: 100px;
    padding-bottom: 40px;
}

#centeredDiv {
    margin: 0 auto;
    width: 200px;
    height: 100px;
    border: 1px solid #000;
}

#centeredParagraph {
    width: 200px;
    margin: 0 auto;
}

#centeredImage {
    display: block;
    width: 200px;
    margin: 0 auto;
}

Result:

JSFiddle example: Centering objects with margin: 0 auto;