Back in December there was a discussion about a tab visibility API. I haven't added this feature to the HTML specification at this time, for a couple of reasons: first, it seems like something we'd really want to have implementation experience before deciding on a specific API, and second, it seems like something that belongs more in the CSSOM API spec than in the primarily media-independent HTML spec.
I have saved all the feedback on the topic in case anyone is interested in working on a specification for this. -- Ian Hickson U+1047E )\._.,--....,'``. fL http://ln.hixie.ch/ U+263A /, _.. \ _\ ;`._ ,. Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.'