For user agents that implement a viewport history mechanism, for each state in a viewport's browsing history, maintain information about the point of regard, content focus, and selection.
When the user returns to any state in the viewport history (e.g., via the "back button"), restore the saved values for the point of regard, content focus, and selection.
Normative inclusions and exclusions
The viewport history associates values for these three state variables (point of regard, content focus, and selection) with a particular document object. If the user returns to a state in the history and the user agent retrieves new content, the user agent is not required to restore the saved values of the three state variables.