CSS: Set a background color which is 50% of the width of the window

Simple solution to achieve "split in two" background:

background: linear-gradient(to left, #ff0000 50%, #0000ff 50%);

You can also use degrees as direction

background: linear-gradient(80deg, #ff0000 50%, #0000ff 50%);

Older Browser Support

If older browser support is a must, so you can't go with multiple backgrounds or gradients, you're probably going to want to do something like this on a spare div element:

#background {
    position: fixed;
    top: 0;
    left: 0;
    width: 50%;
    height: 100%;
    background-color: pink; 
}

Example: http://jsfiddle.net/PLfLW/1704/

The solution uses an extra fixed div that fills half the screen. Since it's fixed, it will remain in position even when your users scroll. You may have to fiddle with some z-indexes later, to make sure your other elements are above the background div, but it shouldn't be too complex.

If you have issues, just make sure the rest of your content has a z-index higher than the background element and you should be good to go.

Modern Browsers

If newer browsers are your only concern, there are a couple other methods you can use:

Linear Gradient:

This is definitely the easiest solution. You can use a linear-gradient in the background property of the body for a variety of effects.

body {
    height: 100%;
    background: linear-gradient(90deg, #FFC0CB 50%, #00FFFF 50%);
}

This causes a hard cutoff at 50% for each color, so there isn't a "gradient" as the name implies. Try experimenting with the "50%" piece of the style to see the different effects you can achieve.

Example: http://jsfiddle.net/v14m59pq/2/

Multiple Backgrounds with background-size:

You can apply a background color to the html element, and then apply a background-image to the body element and use the background-size property to set it to 50% of the page width. This results in a similar effect, though would really only be used over gradients if you happen to be using an image or two.

html {
    height: 100%;
    background-color: cyan;
}

body {
    height: 100%;
    background-image: url('http://i.imgur.com/9HMnxKs.png');
    background-repeat: repeat-y;
    background-size: 50% auto;
}

Example: http://jsfiddle.net/6vhshyxg/2/

EXTRA NOTE: Notice that both the html and body elements are set to height: 100% in the latter examples. This is to make sure that even if your content is smaller than the page, the background will be at least the height of the user's viewport. Without the explicit height, the background effect will only go down as far as your page content. It's also just a good practice in general.

You can make a hard distinction instead of linear gradient by putting the second color to 0%

For instance,

Gradient - background: linear-gradient(80deg, #ff0000 20%, #0000ff 80%);

Hard distinction - background: linear-gradient(80deg, #ff0000 20%, #0000ff 0%);

One way to implement your issue is to add a single line to your div's css:

background-image: linear-gradient(90deg, black 50%, blue 50%);

Here is a demonstration code and more options (horizontal, diagonal, etc.), you can click on "Run code snippet" to see it live.

.abWhiteAndBlack
{
  background-image: linear-gradient(90deg, black 50%, blue 50%);
  height: 300px;
  width: 300px;
  margin-bottom: 80px;
}

.abWhiteAndBlack2
{
  background-image: linear-gradient(180deg, black 50%, blue 50%);
  height: 300px;
  width: 300px;
  margin-bottom: 80px;
}

.abWhiteAndBlack3
{
  background-image: linear-gradient(45deg, black 50%, blue 50%);
  height: 300px;
  width: 300px;
  margin-bottom: 80px;
}

Vertical:

  <div class="abWhiteAndBlack">
  </div>


Horizonal:

  <div class="abWhiteAndBlack2">
    
  </div>


Diagonal:

  <div class="abWhiteAndBlack3">
    
  </div>