Every 2.0s the screen flashes white and a
click plays at the same instant. Record this page full-screen (with system
audio and/or your microphone), then run av-sync-check on the
recording. Append ?interval=1.5 to the URL to change the spacing.
For a camera+mic test: point the webcam at this screen and let the mic hear the speaker. For a screen+system-audio test: record this tab/display with system audio enabled.