Brightness can't be controlled from a webpage. Set it using your monitor or device's own brightness controls before measuring.
At very small sizes (especially 1px), what actually reaches the panel depends on the device: display stream compression, subpixel layout, and browser/OS scaling can all reshape a fine checkerboard before it ever lights up. Sharp single-pixel squares aren't guaranteed on every device.
Slider covers the full range; use −/+ to dial in exact single-pixel rows at the low end.
Tap the stimulus area or this button for a clean fullscreen view with no browser chrome. Use the small × in the upper-left corner to exit; tapping the stimulus itself won't exit, so it can't happen by accident.
System Settings > Displays. True Tone shifts color temperature based on ambient light and will corrupt a controlled color stimulus.
These all bias the color temperature of the whole display away from your intended stimulus color.
Non-native scaling changes the effective pixel geometry, which throws off the estimated pixel-row count below.
The browser's own address bar and tabs sit right next to the test area otherwise. Use the fullscreen button before measuring, and the × in the corner to exit.