Positioning Element at Center of Screen

With transforms being more ubiquitously supported these days, you can do this without knowing the width/height of the popup

.popup {
    position: fixed;
    top: 50%;
    left: 50%;
    -webkit-transform: translate(-50%, -50%);
    transform: translate(-50%, -50%);
}

Easy! JSFiddle here:

Update: Check out for a fairly exhaustive guide on CSS centering. Adding it to this answer as it seems to get a lot of eyeballs.

Set the width and height and you're good.

div {
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  width: 200px;
  height: 200px;
}

If you want the element dimensions to be flexible (and don't care about legacy browsers), go with XwipeoutX's answer.

It will work for any object. Even if you don't know the size:

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

Now, is more easy with HTML 5 and CSS 3:

<!DOCTYPE html>
<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <style>
            body > div {
                position: absolute;
                top: 0;
                bottom: 0;
                left: 0;
                right: 0;

                display: flex;
                flex-wrap: wrap;
                align-items: center;
                justify-content: center;
            }
        </style>
    </head>
    <body>
        <div>
            <div>TODO write content</div>
        </div>
    </body>
</html>

Use flex. Much simpler and will work regardless of your div size:

.center-screen {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  min-height: 100vh;
}

 <html>
 <head>
 </head>
 <body>
 <div class="center-screen">
 I'm in the center
 </div>
 </body>
 </html>

If you have a fixed div just absolute position it at 50% from the top and 50% left and negative margin top and left of half the height and width respectively. Adjust to your needs:

div {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 500px;
    height: 300px;
    margin-left: -250px;
    margin-top: -150px;
}

I would do this in CSS:

div.centered {
  position: fixed; 
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

then in HTML:

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

try this

<table style="height: 100%; left: 0; position: absolute; text-align: center; width: 100%;">
 <tr>
  <td>
   <div style="text-align: left; display: inline-block;">
    Your Html code Here
   </div>
  </td>
 </tr>
</table>

Or this

<div style="height: 100%; left: 0; position: absolute; text-align: center; width: 100%; display: table">
 <div style="display: table-row">
  <div style="display: table-cell; vertical-align:middle;">
   <div style="text-align: left; display: inline-block;">
    Your Html code here
   </div>
  </div>
 </div>
</div>

Now you can do this using grid layout with fewer lines of code.

.center-this{
  display: grid;
  place-items: center;
  align-content: center;
}

<div class="center-this">
  <div>
    Hello
  </div>
</div>

Another easy flexible approach to display block at center: using native text alignment with line-height and text-align.

Solution:

.parent {
    line-height: 100vh;        
    text-align: center;
}

.child {
    display: inline-block;
    vertical-align: middle;
    line-height: 100%;
}

And html sample:

<div class="parent">
  <div class="child">My center block</div>
</div>

We make div.parent fullscreen, and his single child div.child align as text with display: inline-block.

Advantages:

• flexebility, no absolute values
• support resize
• works fine on mobile

Simple example on JSFiddle:

Alternative solution that doesn't use position absolute:
I see a lot of position absolute answers. I couldn't use position absolute without breaking something else. So for those that couldn't as well, here is what I did:
(posted in response to another question).

The solution only focuses on vertically centering because my children elements were already horizontally centered but same could be applied if you couldn't horizontally center them in another way.

If there are anyone looking for a solution,

I found this,

Its the best solution i found yet!

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

Hope you enjoy!

try this:

width:360px;
height:360px;
top: 50%; left: 50%;
margin-top: -160px; /* ( ( width / 2 ) * -1 ) */
margin-left: -160px; /* ( ( height / 2 ) * -1 ) */
position:absolute;