The test runs entirely in your browser. It starts with your first character and stops when time runs out or you finish the passage. Pasting is blocked, but Backspace works, so you can correct mistakes as you type. Your text and result stay on your device.
WPM uses the standard convention of five correct characters per word, including spaces and punctuation. That makes scores comparable even when the passage contains words of different lengths. Accuracy is the percentage of your currently typed characters that match the passage.