Browser tabs have revolutionized the way we browse the Web, but they've given us an tricky problem: developers can't tell if the tab their site is displayed in is being viewed. To solve that problem we have the Visibility API, which allows us to test whether the browser content can be seen, and respond accordingly. It's ideal for sites that play music for example, to pause the playback whilst the user is away. The Page Visibility API comes with several properties: document.hidden: This property returns...