HTML is a markup language whose visual presentation was initially left up to the user. However, as the Internet expanded from the academic and research world into the mainstream in the mid-1990s, and became more media oriented, graphic designers sought ways to control the visual appearance of their Web pages. As popularised especially by the designer David Siegel in his book "Creating Killer Web sites", tables and spacers (usually transparent single pixel GIF images with explicitly specified width, height or margins) were used to create and maintain page layouts.