What the monitor sharpness test shows
If you can see structure — a pattern, a shimmer, or a colour tint — something in the chain is not showing the image one pixel to one pixel. Either the display is scaling, or a sharpness setting is adding edges that are not in the signal.
This is drawn on device pixels rather than CSS pixels. That distinction matters enormously: a "one pixel" pattern drawn in CSS pixels on a 2× display is two device pixels wide and tests nothing at all, which is what most versions of this test do.
How to read the sharpness pattern
Sit at your normal viewing distance. The checkerboard should blend into an even grey. Step closer and you should see the individual pixels; step back and it should be uniform.
A visible shimmer or moiré usually means the display is scaling. A bright or dark halo means the sharpness control is too high — most displays look best with sharpness at or near zero, and the factory default is usually too aggressive.
The convergence pattern draws red, green and blue lines one pixel apart. On an LCD they should be perfectly parallel and distinct.
What this test cannot tell you
Only valid at 100% browser zoom and at the panel’s native resolution. Any scaling in between makes the pattern meaningless — and on a HiDPI display, the pattern is drawn at device resolution, so it will look finer than you expect.