Pick a color field and it takes the whole screen.
A dark dot on white is a dead pixel; a bright dot on black is a stuck one.
Pick a field below, or press Start to walk all ten.
Turn the brightness up first, and clean the glass. Dust looks exactly like a dead pixel until you wipe it and it moves.
Each field is chosen to expose a different fault. The list of what to look for on each one is below the test. This browser cannot keep the display awake, so set your screen timeout generously before you start.
Pick a field and it fills the screen with one flat color. A dark dot on the white field is a pixel that never lights up, and a bright dot on the black field is one stuck on. The red, green and blue fields narrow it to a single subpixel, and the gray and gradient fields show uneven backlighting and banding. Clean the glass first, because dust moves and a dead pixel does not.
Run the white field and look at the mark, then wipe the glass with a dry microfibre cloth and look again. Dust moves or disappears. A dead pixel sits in the same place forever, and it is square. Dust is not.
The other giveaway is the black field. Dust on a black screen is invisible; a stuck pixel is a bright point you cannot miss.
A dead pixel receives no power and stays black, so it shows on the white field and on all three primaries. A stuck pixel is permanently on at one color, so it shows on the black field as a colored point.
Which one you have decides whether it is worth trying anything. A stuck pixel sometimes clears on its own or after the panel has been running a while. A dead one does not come back.
Each pixel is built from separate red, green and blue elements, and a fault often affects one of them rather than the whole pixel. On the red field, a dark dot means that pixel's red element is out while the other two still work.
The cyan, magenta and yellow fields do the reverse: two channels on and one off. A colored dot there is an element stuck on rather than one that has failed off. On a panel whose subpixels are not simple RGB stripes, and many OLED layouts are not, the fields still find the defect. They just stop naming which element it is.
Because a browser reports CSS pixels, not the panel's pixels. The page multiplies by the device pixel ratio to show both numbers, and that ratio mixes your system's display scaling with your browser's zoom into one value that no interface separates.
So a 4K monitor at 150% scaling reports 2560 across, and the page cannot tell whether that came from Windows or from a Ctrl+ you pressed last week. Setting the browser back to 100% zoom brings the two numbers closer to the panel.
No interface reports the display's refresh rate, so this page counts something adjacent: how many times a second the browser actually paints. That number follows the display when nothing is in the way.
Plenty gets in the way. A compositor saving power, a window straddling two monitors with different rates, or a browser capped at 60 all report low while the monitor runs at 144. Read the figure as a floor under your refresh rate, never as a reading of it.
The same thing, and phone screens get dead pixels and image retention like any other. The fields cover the viewport, and where the browser grants fullscreen they cover the display.
On an iPhone, fullscreen for an ordinary element is still partial, so the browser's own bars may stay on screen and the strip beneath them never gets a field. The test says so when that happens. Keeping the screen awake needs iOS 18.4 or newer; below that, set your auto-lock generously before you start.
This test's engine last changed on . The page says only what the engine can do, and the two change together.