Prior to the adoption of Cascading Style Sheets (CSS), the spacer GIF was a transparent image, often used to control blank space within a web page, that can be resized according to the width and height dimensions it is given. Spacer GIFs are not browser specific. The reason a spacer GIF is invisible is so that an HTML developer can create a table cell and fill the background with a specific color that can be viewed through the transparent spacer GIF. For instance, let's say the developer wants to create a blue box that is 500 x 500 pixels wide and high. Instead of using a separate blue graphic that is 500 x 500 pixels (taking up additional bandwidth), the developer can specify the table cell background color and specify the dimensions of the spacer GIF that already exists.
History
David Siegel's 1996 book Creating Killer Web Siteshttp://www.killersites.com/killerSites/core.html was allegedly the first to publish the Spacer GIF technique. According to Siegel, he invented the trick in his living room. Interestingly, others were probably inventing the same trick at around the same time (maybe).[1]
The use of spacer GIFs has declined due to the prevalence of Cascading Style Sheets (CSS) for laying out web pages, which achieves the same effect by changing the margin or padding on a given element. If used properly, CSS reduces unnecessary code in a web page. Blank 1x1 GIFs are still occasionally used to fix a PNG rendering limitation in Microsoft Internet Explorer versions 5.5 - 6.
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