Size helps test the resolution, aspect ratio and pixel density of your web browser.

The screen is divided into four equal parts, each representing a viewport-sized quarter. The top left quarter shows live screen and viewport diagnostics. The other 3 quarters display 16:9 images of vertical bars. The bars alternate between black and white and are arranged in such a way that they will only be fully visible at native 4K (3840x2160), 1080p (1920x1080) and 720p (1280x720) resolutions respectively.
If there are borders around the bars, the browser is not at 16:9. If the bars are not clearly visible, for example if they are a gray or have moiré patterns, the browser is not at the native resolution for that image.