How the measurement works
Browsers draw animations in step with the display: every time the screen refreshes, the page gets one chance to paint a frame. The test asks for a frame again and again and records the time between them. On a 60 Hz screen that interval is 16.67 ms, at 144 Hz it is 6.94 ms. The typical interval is taken from the middle of the measurements, so a few slow frames do not drag the result down, and the value is matched to a common refresh rate when it is within 3 percent. Many 60 Hz screens actually run at 59.94 Hz, which is shown as 60.
Each dropped frame is counted when an interval is clearly longer than normal. A few drops while you scroll or switch tabs are expected; regular drops while nothing else happens point to a busy computer, a power saving mode or a browser extension.
Seeing 60 Hz on a faster monitor?
Check the display settings: on Windows under Settings, System, Display, Advanced display; on macOS under System Settings, Displays. The rate is often left at 60 Hz after setup.
Use a cable that can carry the rate: DisplayPort, or HDMI 2.0 or newer for high rates at higher resolutions. Older HDMI cables and adapters often limit you to 60 Hz.
Laptops on battery, and power saving modes, may lower the rate. Plug in and test again.
With two monitors, the browser follows the screen the window is on. Move the window to the fast screen and reload.
Variable refresh rate (G-SYNC, FreeSync, Adaptive Sync) usually still shows the maximum rate here, because a browser animation runs at full speed.
Reading the motion test
Each lane moves an object by the same number of pixels every frame. At 60 Hz the fastest one jumps 16 pixels per frame, at 240 Hz only 4, so higher rates look smoother and sharper. If the motion stutters at regular intervals, frames are being dropped. A trail behind the object is blur from the panel and from your eyes following it, and is normal to some degree on every LCD and OLED screen.