 Modern browsers provide an optimization feature for history navigation called the Backward Forward Cache, BF Cache for short. It enables an instant loading experience when users go back to a page they have already visited. Pages can be blocked from entering the BF Cache or get evicted while in BF Cache for different reasons, some required by specification and some specific to browser implementations. Previously, there was no way for developers to find out why their pages were blocked from using the BF Cache in the field. Though there was a test in Chrome DevTools to enable monitoring in the field, the performance navigation timing class has been extended to include a Not Restore Reasons property. This returns an object containing related information on all frames present in the document.