Test Mode: High Speed Camera — To compare display refresh behavior.
This test needs a camera with a high-speed-video function (480fps+). An example recording is High Speed Video of LightBoost. This test is useful for capture display refreshing behaviour, including scanout behavior. Use full screen mode with caution.

Test Mode: Oscillscope — To measure display GtG pixel response.
This mode is good for photodiode oscilloscopes. It slows down the flicker to your preferred flicker rate. This is useful because LCD pixel response can overlap multiple refresh cycles. See Pixel Response FAQ: GtG versus MPRT

Test Mode: Mouse Latency - To compare latency of same-system changes.
Test lag of different computer setups via a high speed camera filming both the screen and a fast press onto mouse button. This test flashes upon mouse click. With a high speed camera, this can measure relative latency differences between systems and/or settings changes. There are many error margins including browswer lag, graphics driver lag, display lag, refresh interval granularity, camera frame rate granularity, display scanout latency, and even windowed versus full screen mode. For this reason, compare only via many test passes only on the same system, while changing one variable (i.e. a different browser or a different mouse, to compare lag differences). Inspired by this tweet.

WARNING: Extreme flicker for photosensitive individuals!
Temporary image retention may occur during 'High Speed Camera' mode on some LCDs.