Choose a useful pattern
Try a block, circle, star or subtitle-like pattern. A smaller bright shape can reveal a different relationship with the surrounding dark area from a large shape. Change one parameter at a time so the comparison remains understandable.
Local dimming and the viewing environment
A halo can depend on local-dimming behavior, the selected picture mode and your viewing angle. Room light and camera exposure change how visible it looks. Compare the synthetic pattern with real subtitles or other familiar content before deciding whether the effect is distracting.
What the pattern cannot establish
A browser cannot count dimming zones or assign a hardware display rating by observing its own image. The shape sizes here are CSS dimensions, not a map of the backlight. Use them to describe behavior and document settings, alongside the device's specifications and a suitable review method.