Geographic Information Systems (GIS) are essential tools for many epidemiologists, public health advocates, and others in the health sector such as insurance adjusters and emergency responders. Public health practitioners are often concerned with accessibility. Accessibility also refers to ensuring results and maps are accessible to those with visual impairment. As maps are an inherently visual medium, this typically involves including as much semantic and descriptive information encoded into GIS as possible, so that screenreading programs can accurately reproduce information encoded in a map through speech synthesis programs. A "cartogram" is a popular data visualization technique that distorts areas of a map based on the intensity of values. Prior to computerized GIS, cartograms were constructed painstakingly by hand. Of course, many GIS applications allow styling and formatting without using any Hypertext Markup Language (HTML) code, but understanding basic HTML allows more advanced customization.