Check your monitor, laptop or phone screen for dead or stuck pixels. Fill the screen with solid colors and look closely for spots that do not match.
Click Start test, then click anywhere (or press the right arrow or space) to cycle through colors. Look closely for dots that stay a different colour. Press Esc or the Exit hint to leave.
A stuck pixel is permanently lit in one colour (red, green or blue) and often shows up on a black background. A dead pixel stays black and shows up best on white or bright colours. Cycling through solid colors helps you spot both.
Click Start test to fill the screen with a solid colour, then click to cycle through red, green, blue, white, black and more. Look closely (a magnifier helps) for any dot that does not match the rest of the screen.
Not directly, but rapidly flashing colors on the area sometimes helps a stuck pixel recover. A dead pixel (always black) usually cannot be fixed in software.
It tries to go fullscreen so nothing is hidden behind browser bars. If your browser blocks that, the colour still fills the whole window.