When websites want to know what browser you're using, they often examine the "user agent", or "UA" string. This is a string that provides information about what browser and operating system you're using. Beginning with Chrome 11, we're making some changes to our UA string, which can affect website compatibility.

For reference, here is what the current (Chrome 10) UA string looks like on a few different platforms:

Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16

Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.204 Safari/534.16

And in comparison, here are the UA strings for Chrome 11 on the same platforms:

Mozilla/5.0 (Windows NT 6.0; WOW64) AppleWebKit/534.24 (KHTML, like Gecko) Chrome/11.0.696.16 Safari/534.24

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.24 (KHTML, like Gecko) Chrome/11.0.696.16 Safari/534.24

Let's break down the differences in detail. We've made four changes, two of which are Windows-specific:

1. On Windows, the initial "Windows;" platform identifier has been removed. This was redundant with the subsequent OS version identifier, and makes us more compatible with Internet Explorer, whose UA string doesn't have this initial token.
2. The "U" SSL encryption strength token has been removed. This token dates from more than a decade ago, when U.S. export laws limited the encryption strength that could be built into software shipped to various other countries; the valid values are "U" (for "USA" 128-bit encryption support), "I" (for "International" 40-bit encryption support), and "N" (for "None", no encryption support). These days, every browser ships with 128-bit SSL support everywhere, so it's not necessary to advertise it.
3. On 64-bit versions of Windows, tokens have been added after the OS version. For 32-bit Chrome builds running on 64-bit Windows, we've added "WOW64". (WOW64" stands for "Windows 32-bit On Windows 64-bit" and is the name Microsoft gives its 32-bit compatibility subsystem.) In our source code, we've also added identifiers for 64-bit native builds, specifically "Win64; x64" for x64-based processors and "Win64; IA64" for Itanium systems. (However, we don't currently ship such builds, or have any immediate plans to.) These tokens are useful for sites that need to provide download links for native executables, and match what Internet Explorer uses.
4. The locale has been removed. Web authors who want to know what languages a browser supports should use the HTTP Accept-Language header instead, which can supply multiple locales. In fact, websites that relied on the UA string locale probably had some very unhappy visitors, because Chrome always had a bug where the UA string locale was reported as "en-US", regardless of the user's desired locale(s)!

One more question remains: why are we making these changes now? Because websites tend to use common pieces of code to check all browsers' UA strings, it's important for browsers to stay in sync with each other. Mozilla has made the above changes in Firefox 4 (and more; see http://hacks.mozilla.org/2010/09/final-user-agent-string-for-firefox-4/ for details), which was released recently, and we wanted to change Chrome to match as soon as possible, to minimize the disruption to web authors.

As the changes above have trickled into our Canary and Dev builds, we've already found and fixed some problems in Google's UA string parsing libraries that have caused compatibility issues with Google sites (though not all of the affected sites have updated yet). If you see problems on other sites you think might be caused by the new UA string, try running Chrome with an alternate UA string using the --user-agent="<Put older UA string here>" command line flag. (You can double-check the UA string Chrome sends to websites by typing about: in your address bar and hitting <enter>.) If that fixes the problem, please let us know by filing a bug in our bug tracker at http://crbug.com/.

The web is becoming more interactive and animated day by day. Many web pages use the Canvas element to draw rich 2D content via the 2D context or modify DOM elements on the fly. These pages generally use the setTimeout or setInterval APIs to receive frequent callbacks, allowing them to redraw their content periodically, or use DHTML to move elements on the page. As 3D content drawn using the WebGL API increases in popularity, it will use similar animation techniques.

Unfortunately, setTimeout and setInterval don’t take into consideration whether the destination element, or even the tab that contains it, is actually visible. So, pages with high-frequency timers will consume CPU resources even if the tab is in the background. On laptops, netbooks, and mobile devices of all kinds, reducing CPU consumption is essential in order to prolong battery life. Additionally, excess CPU consumption by background tabs reduces the smoothness of animations on the foreground tab.

Excessive CPU consumption by timers on web pages is not a theoretical problem. We have measured web sites containing mostly static text content firing timers at a rate of over two hundred per second.

Mozilla recently introduced the experimental mozRequestAnimationFrame API, which has different semantics than setTimeout or setInterval. Instead of the developer specifying a target frame rate, the browser runs the given callback when it is ready to produce the next animated frame. The callbacks are specifically known to be relevant to the animation of the page, and don’t run too often.

An experimental webkitRequestAnimationFrame API has been upstreamed to WebKit, and is available starting in Chrome 10. This is essentially the same as mozRequestAnimationFrame, but supports an optional second argument which is the element that the callback intends to animate. This additional information will allow the browser to avoid animating elements that are not visible to the user. See this bug report for more details. Chrome doesn’t run requestAnimationFrame callbacks for background tabs at all, which dramatically reduces CPU consumption when multiple tabs containing animated content are in the same window.

The WebGL samples project contains a three dimensional graphics library that has been modified to use requestAnimationFrame rather than setTimeout or setInterval. Take a look at this library for a good example of how to convert existing timeout based animations to the new style, while preserving compatibility with browsers that don’t support requestAnimationFrame.

In the forthcoming Chrome 11 release, we plan to reduce CPU consumption even for pages that are using setTimeout and setInterval. For background tabs, we intend to run each independent timer no more than once per second. This change has already been implemented in the Chrome dev channel and canary builds. While there may be some compatibility impact for web pages, we believe that improving the user experience for the foreground tab, and increasing battery life, are problems needing to be addressed. Please send us your comments on this planned change.

Posted by Kenneth Russell, Software Engineer

Posted by Chris Evans, Google Chrome Security Team, Bernhard Bauer, Software Engineer, and Carlos Pizano, Software Engineer

Over the last few months, we’ve made a lot of progress using graphics hardware (commonly referred to as the GPU) to make Chrome faster and more power-efficient. However, as we’ve rolled out features like WebGL and GPU-accelerated HTML5 video, we noticed a troubling trend: users with old graphics drivers experienced a significant increase in crashes when using these features. Because stability is one of Google Chrome’s core principles, we’ve recently become stricter about requiring up-to-date drivers and graphics hardware by adding ranges of old drivers to Google Chrome’s software rendering list.

Developers should continue to ensure that the software-rendered version of their sites work properly for users without GPU-accelerated browsers, so we expect most content to continue to function normally for Google Chrome users with out-of-date drivers -- albeit, without the same performance you might expect from Chrome. WebGL content on out-of-date systems will currently not display, but we are working to provide a software path so that these systems can run basic 3D applications.

As our ability to determine whether a machine can reliably use GPU features improves, we hope to extend hardware acceleration support to more and more users. Here are some steps you can take to maximize the chances that Chrome will run fully hardware-accelerated on your computer:

1. Use the latest major version of your operating system (such as Windows 7 or Mac OS 10.6)
   
2. Install all system updates and driver updates that are available for your system.
   

Posted by Henry Bridge, Product Manager