At what point does the navigation start timing occur?

The navigation start timing occurs after the unload prompt terminates on a previous document. This timing marks the end of the current page's lifecycle and indicates that the browser is ready to begin loading a new page. The unload event is fired when the user navigates away from the current document, signaling that the resources for that document are no longer needed. Once this prompt has been completed, the navigation timing can accurately reflect the performance and timing metrics for the new document being loaded. This allows for an accurate assessment of how long it takes for the new page to load and become interactive, which is essential for understanding user experience and web performance.

Other options, while relevant to the web navigation process, do not define the specific point at which navigation timing starts. For instance, timing does not begin with the first click on a document as that does not account for the lifecycle completion of the previous document. Similarly, navigating to a new tab or after all resources have finished loading does not reflect the correct moment to start measuring the timing of navigation for performance metrics.