Units of Measurement

• CSS uses different measurements for describing width. The most common measurements used in CSS are pixels, designated with "px," and percentages, designated with "%." Pixels are static -- 100 pixels is 100 pixels across all platforms, regardless of the user's resolution -- while percentages are flexible. Pixels allow the designer to control the appearance of a website, but percentages make the website accessible to all users.
  
  

Using Pixels

• When using pixels, the definition of "full width" varies from resolution to resolution. According to StatCounter, the most common resolution worldwide is 1024 by 768. A website that makes all parent elements 1024 in width could be considered a "full-width" CSS layout, but only for screens with a 1024 by 768 resolution. In North America, the most common resolution is 1280 by 800; on these computers, the "most common" layout wouldn't reach the full width of the screen.
  
  

Using Percentages

• Percentages should be used when creating a full-width layout that adjusts itself automatically on all machines. Websites that use percentages are fluid, changing according to the visitor's resolution; 10 percent could equal 80 pixels on one screen and 136 on another. To make a full-width layout in CSS, use "100%" when assigning width to parent elements that are meant to stretch across the entire page. For example, to make a header that stretches across the full width of the monitor, you might put the following into your code:
  
  <style>
  .header { width: 100%; }
  </style>
  
  Then, in your HTML, you'd put "<div class="header">.
  
  

Eliminating Margins

• Margins are additional white space that surround the top, bottom, right and left side of a page or document. Margins are used to make pages easier to read and should be used in the majority of cases. However, if you want a layout that really uses the entire width of the window, you can put the following in between the "<style>" tags in your CSS code:
  
  body { margin: 0px; }
  
  The above code zeroes out the margins on all sides of the Web page.