Note: this has only been tested on Firefox browsers -- Chrome works, but does not always reach 240 Hz.
Supplemental website for the HPG 2019 short paper: Temporally Dense Ray Tracing by Pontus Andersson, Jim Nilsson, Marco Salvi, Josef Spjut, and Tomas Akenine-Möller.

Hang on, measuring monitor refresh rate...

Explanation: In this test, we show four images after each other, and they have been sampled with one sample per pixel, where each image uses a constant sample offset according to the 4x MSAA pattern (sometimes called RGSS). The effect is that, in 30 Hz, the images are jumping around, but at 240 Hz, they integrate into a perceived effect of 4x MSAA. This (magically) happens due to the integration effect of the human visual system, the monitor and the four images.

Desired frame rate: