Text size, line breaks, graphics positions, are variable. The viewer can grossly affect the design by browser window size and browser text size settings. Images take longer to download, and some browsers don't display text at all. Readability of text can be enhanced by using a table format to control line length of text on a page. Inserting a table and defining the fixed width in pixels keeps the text from spreading all the way across the entire screen.
Text is more readable as a dark image on a light or white background.
Use of white letters on black or a dark color should be limited to display or headlines for dramatic effect. It is just plain hard to read a paragraph in white on black. Although it may be readable on your monitor, some viewers will not be able to discern the letters from the background pattern. Use animation, sound, and movies sparingly. Continuous animation, winking, blinking and gyrating graphics can be tiring and distracting to viewers trying to get information from your page.