This page demonstrates the Page Visibility API.
The API can be used to check if and when a web page is visible.
When you minimise this page or move to another tab, JavaScript detects a visibilitychange event. The video pauses and the page title stops changing – and vice versa if you return to the page.
Please note: this demo requires a browser such as Firefox or Google Chrome that supports the Page Visibility API.
For more information, please see my blog at samdutton.wordpress.com
To detect element visibility (rather than page visibility) take a look at the jquery.inview plugin.