^⌥⌘-8 (Control-Option-Command-8) key combination inverts the display screen color to white on black. If you’re used to reading black text on white background, reading a site with a white text on black background might be uncomfortable for you. This feature might then be useful.
Pressing the same key combination will toggle back the display colors.
You can find the settings for this feature under Preference>Universal Access.
As seen from the Preference screen, you can choose to display grayscale for the display.