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Velocity Google has urged website developers to use Chrome's experimental Page Visibility API to reduce their sites' activities when they're not actually being viewed by browser users.

Now included with the developer version of Chrome – and due for arrival in the beta version next week – the PageVisability API allows websites to determine when they're actually being viewed by users – and when they're just sitting in the background. The API can tell you when a site is sitting inside a background tab, but also when a site has been pre-rendered by Chrome's new Instant Pages tool.

But webmasters can also use the API to determine when a site is sitting in a background tab – Instant Pages pre-renders sites in what is essentially a hidden tab – and on Thursday, at O'Reilly's Velocity conference in Santa Clara, California, Google engineers Arvind Jain, Sreeram Ramachandran, and Mike Belshe asked developers to use the API as a means of scaling back their site's operations when they're relegated to the background.

"We all build our webpages on our own, and we try to take the resources that we need to run our webpages," Belshe said. "And that's what we should be doing. When we're in the background, if we can recognize that other pages are running too and knock our own stuff down, that's really the way to go".

Mike Belshe

Google has proposed the API to the W3C as a standard, and it hopes that other browser vendors will adopt it as well. After Belshe's talk, Mozilla open source evangelist Chris Blizzard told us that he and Mozilla are still mulling the ramifications of the API.

With Chrome, Belshe said, Google already worked to downgrade activity in background tab. He called this the "real" reason Chrome is so fast. "For things that aren't running [in the foreground tab], we knock them to a lower priority. Not only does that make sure that Chrome runs better, it makes it so that Outlook or Word or whatever else runs better," he said.

But he added that website owners can improve performance even more by "knocking down" their own activity when their sites are running in the background. "If you've got a webpage that's doing stuff constantly, you tap into the Page Visibility API, and you slow it down. You don't have to stop. Just slow it down."

Belshe ran a benchmark showing that Chrome is significantly faster than Microsoft Internet Explorer when heavy JavaScript is running in multiple tabs, saying this was the result of Google's work to reduce activity in background tabs. Blizzard told us that Mozilla is treating background sites in similar fashion with Firefox 5, due for release next week. ®

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