Note: On macOS specifically, you may need to enable certain settings for keyboard navigation to work as expected. Go to your Mac Settings, click "Keyboard", click "Shortcuts" and check "Use keyboard navigation to move focus between controls." On Mac Ventura, go to System Settings > Keyboard Navigation and turn Keyboard navigation on (to use keyboard navigation to move focus between controls).
When using the TAB key to navigate through the page, make note of any situation where you have to hit TAB within a certain window of time to activate. If for example, you can only interact with an element for 5 seconds before the element disappears or can't be interacted with anymore, that would constitute a failure of this criterion.
This guideline is not looking for situations where certain elements are hidden away until an explicit action is taken by the user. For example, if you activate a modal with the keyboard, new focusable elements may appear that weren't available before. But, as long as the modal does not disappear on its own, the user has no time constraint in interacting with the modal.
