They became mostly obsolete after the browser wars-fueled addition of layout attributes to HTML 2.0 table tags, and were mostly unused by the time Cascading Style Sheets became widely adopted.
[1][2] The Cascading Style Sheets (CSS) standard 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.
Prior to the widespread adoption of Cascading Style Sheets (CSS), the spacer GIFs were used to control blank space within a web page, that can be resized according to the HTML attributes it is given.
Different screen resolution, browser rendering engines, as well as user font preferences, could change the layout of the design considerably.
Although CSS' adoption was slow, owing to poor browser implementations and developer inertia, the basic ability to control element placement as enabled by the use of spacer GIFs was usable by about 1997.