This test is very difficult to accurately adjust in bright lighting. It is easier to calibrate in total darkness at night, or in a totally dark room, especially if using 10-bit mode. When testing a mobile display (e.g. laptop), bring it to a dark room such as an interior room. Long-exposure photography may also be used. Also useful for OLED banding tests.
This test also has a white level (Clipping) mode, for calibrating your white level clipping. Calibrating both black and white levels will help maximize color gamut without clipping. This test supports both SDR (sRGB) for 8-bit greyscale, and HDR (Rec.2020) for 10-bit greyscale. 10-bit can test for banding artifacts in browser / OS / display chain.
You can click/tap the screen to see square boundaries briefly appear; these are simply guide lines to let you know where they are on the screen. The guide lines are there to assist you in locating difficult-to-find rectangles, and are not part of the calibration. Double clicking will enter full screen mode.