Open the WebKit Nightly ...
... from source: execute
... from binary: open the package and run WebKitNightly.app
In the Menubar, go to "Safari > Preferences..." (Cmd+,). Go to the "Advanced" tab, and check the option "Show Develop in menu bar" if it's not checked already.
Ensure that "Develop > Use WebKit Web Inspector" is not selected.
Open the inspector from "Develop > Show Web Inspector" (Cmd+Opt+I)
A "movie camera" icon should be visible in the top dashboard. Click it to expand the Timelapse overview. See Using Timelapse for instructions on using Timelapse functionality.