Purpose
A mark that appears only on red, or that changes colour here, is worth writing down. It could be a subpixel that does not match the others, or dust that happens to show more on this field. One colour is not a verdict.
Preparation
Ordinary room light. Note filters if they are on. Do not press the panel. For edge glow, switch to the black field and dim the room.
What to look for
Anything that is not the flat field: a dot, a stain, a tint, a bright edge. The value on this page is #FF0000. Fullscreen uses that same value with no logo and no colour filter.
Common false positives
Dust, reflections, a protector, and judging from a photo. Scaling means a CSS pixel is not guaranteed to be one panel pixel.
Limitations
The browser asks the display to show the value. It cannot read back what the panel emitted. There is no pass or fail.
Questions
Is this red a calibrated reference?
No. It is an exact 8-bit sRGB field in the page. The display’s own settings change how it looks. It is not a white point, a meter reading or a profile.
Why is there a frame around the preview?
The frame is outside the field so the colour stays pure. Fullscreen removes the frame. If the browser will not enter fullscreen, the same colour still fills the view.
Can I save the field?
Yes. Download a PNG, or use the files on the downloads page. The file is a generated pattern, not a photo of your panel.
Does a mark on this colour prove a fault?
No. Compare it on white, black and the other primaries, and see whether a dry wipe moves it. Write down what you see.