Originally, computer user interface images were formed on CRTs. The phosphor was normally a very dark color, and lit up brightly when the electron beam hit it, appearing to be green or amber on black, depending on phosphors applied on a monochrome screen. RGB screens continued along a similar vein, using all the beams set to "on" to form white.
With the advent of teletext, research was done into which primary and secondary light colors and combinations worked best for this new medium. Cyan or yellow on black was typically found to be optimal from a palette of black, red, green, yellow, blue, magenta, cyan and white.
Some argue that a color scheme with light text on a dark background is easier to read on the screen, because the lower brightness causes less eyestrain. The caveat is that most pages on the web are designed for white backgrounds; GIF and PNG images with a transparency bit instead of alpha channels tend to show up with choppy outlines, as well as causing problems with other graphical elements.
It is not necessary that a web design work well with only one color scheme. There are many mechanisms of web architecture that allow designs to work well with any color scheme a user might prefer. This technical flexibility is a product of the web architect's concern for accessibility and user preference empowerment, though designers rarely utilize this technical flexibility. Users who prefer certain color schemes frequently apply their preference to web pages using tools such as Stylish.
Blazing white is black text on a bright background found in some software packages, often without the option to set colors (e.g. Skype, or Wikipedia). Another common problem is, when using spatial anti-aliasing, the software assumes the background color is white.
Unlike paper, which reflects ambient light, both CRT and LCD displays emit light of sufficient brightness to overcome ambient light. As ambient light varies, the relative brightness of the display can vary widely.
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