A simple approach that worked for me to horizontally center a block of unknown width:

<div id="wrapper">
  <div id="block"></div>
</div>

#wrapper { position: absolute; width: 100%; text-align: center; }
#block { display: inline-block; }

A text-align property may be added to the #block ruleset to align its content independently of the alignment of the block.

This worked on recent versions of Firefox, Chrome, IE, Edge and Safari.

You can place the image in a div and add a div id and have the CSS for that div have a text-align:center

HTML:

<div id="intro_img">

    <img src="???" alt="???">

</div>

CSS :

#intro_img {
    text-align:center;
}

.center {
  position: absolute
  left: 50%;
  bottom: 5px;
}

.center:before {
    content: '';
    display: inline-block;
    margin-left: -50%;
}

HTML:

<div class="wrapper">
    <div class="inner">
        content
    </div>
</div>

CSS:

.wrapper {
    position: relative;

    width: 200px;
    height: 200px;

    background: #ddd;
}

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

    width: 100px;
    height: 100px;

    background: #ccc;
}

This and more examples here

This solution works if the element has width and height

.wrapper {
  width: 300px;
  height: 200px;
  background-color: tomato;
  position: relative;
}

.content {
  width: 100px;
  height: 100px;
  background-color: deepskyblue;
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  margin: auto;
}

<div class="wrapper">
  <div class="content"></div>
</div>

#container
{
  position: relative;
  width: 100%;
  float:left
}
#container .item
{
  width: 50%;
  position: absolute;
  margin: auto;
  left: 0;
  right: 0;
}

sass/compass version of Responsive Solution above:

#content {
  position: absolute;
  left: 50%;
  top: 50%;
  @include vendor(transform, translate(-50%, -50%));
}

My preferred centering method:

position: absolute;
margin: auto;
width: x%

• absolute block element positioning
• margin auto
• same left/right, top/bottom

JSFiddle here

This worked for me :

<div class="container><p>My text</p></div>

.container{
position: absolute;
left: 0;
right: 0;
margin-left: auto;
margin-right: auto;
}

Flex can be used to center absolute positioned div.

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

.relative {
  width: 275px;
  height: 200px;
  background: royalblue;
  color: white;
  margin: auto;
  position: relative;
}

.absolute-block {
  position: absolute;
  height: 36px;
  background: orange;
  padding: 0px 10px;
  bottom: -5%;
  border: 1px solid black;
}

.center-text {
  display: flex;
  justify-content: center;
  align-items: center;
  box-shadow: 1px 2px 10px 2px rgba(0, 0, 0, 0.3);
}

<div class="relative center-text">
  Relative Block
  <div class="absolute-block center-text">Absolute Block</div>
</div>

** RESPONSIVE SOLUTION **

Assuming the element in the div, is another div...

this solution works fine

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

the container can be any size(must be position relative)

.container {
position: relative;/*important*/
width: 200px;/*any width*/
height: 200px;/*any height*/
background: red;
}

The element(div) can also be any size(must be smaller than container)

.center {
position: absolute;/*important*/
top: 50%;/*position Y halfway in*/
left: 50%;/*position X halfway in*/
transform: translate(-50%,-50%);/*move it halfway back(x,y)*/
width: 100px;/*any width*/
height: 100px;/*any height*/
background: blue;
}

The result will look like this. Run the code snippet

.container {
	position: relative;
	width: 200px;
	height: 200px;
	background: red;
}

.center {
	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate(-50%,-50%);
	width: 100px;
	height: 100px;
	background: blue;
}

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

I found it very helpfull

This is a mix of other answers, which worked for us:

.el {
   position: absolute;
   top: 50%;
   margin: auto;
   transform: translate(-50%, -50%);
}

Works on any random unknown width of the absolute positioned element you want to have in the centre of your container element.

Demo

<div class="container">
  <div class="box">
    <img src="https://picsum.photos/200/300/?random" alt="">
  </div>
</div>

.container {
  position: relative;
  width: 100%;
}

.box {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  display: flex;
  justify-content: center;
  align-items: center;
}

If you need to center horizontally and vertically too:

left: 50%;
top: 50%;
transform: translate(-50%, -50%);

Searching for an solution I got answers above and could make content centered with Matthias Weiler answer but using text-align.

#content{
  position:absolute;
  left:0;
  right:0;
  text-align: center;
}

Worked with chrome and Firefox.

this work for vertical and horizontal

  #myContent{
        position: absolute;
        left: 0;
        right: 0;
        top:0;
        bottom:0;
        margin: auto;
   }

and if you want make element center of parent, set position of parent relative

 #parentElement{
      position:relative
  }

edit:

• for vertical center align set height to your element. thanks to @Raul

• if you want make element center of parent, set position of parent to relative

Absolute Centre

HTML :

<div class="parent">
  <div class="child">
    <!-- content -->
  </div>
</div>

CSS :

.parent {
  position: relative;
}

.child {
  position: absolute;

  top: 0;
  right: 0;
  bottom: 0;
  left: 0;

  margin: auto;
}

Demo: http://jsbin.com/rexuk/2/

Tested in Google Chrome, Firefox, and IE8

Hope this helps :)

Really nice post.. Just wanted to add if someone wants to do it with single div tag then here the way out:

Taking width as 900px.

#styleName {
    position: absolute;
    left: 50%;
    width: 900px;
    margin-left: -450px;
}

In this case one should know the width beforehand.