Prior to the widespread 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 HTML attributes it is given. 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, a developer seeking to create a square blue box 500 pixels on a side could use a separate blue 500×500 graphic at the expense of additional bandwidth. Instead, the developer can specify the table cell background color and specify the dimensions of a pre-existing transparent spacer GIF.
David Siegel's 1996 book Creating Killer Web Sites was allegedly the first to publish the Spacer GIF technique. According to Siegel, he invented the trick in his living room.
The Cascading Style Sheets (CSS) standard has diminished the use of spacer GIFs for laying out web pages. CSS can achieve the same effects in a number of ways, such as by changing the margin or padding on a given element or by explicitly setting a relative position. If used properly, CSS reduces unnecessary code in a web page. Blank 1×1 GIFs are still occasionally used to fix a PNG rendering limitation in Microsoft Internet Explorer versions 5.5 – 6.