Microsoft on Wednesday unveiled the second test version of its next Web browser, Internet Explorer 10. This will be the stock browser to ship with the highly anticipated Windows 8 operating system, expected next year, and today's release is actually the same IE engine used in Microsoft's first , and will serve as the engine powering newfangled apps on that OS.

Internet Explorer 10 Platform Preview 2, as this release is formally known, implements several new HTML5 capabilities, turbocharges HTML5 performance, and adds some security options for Web applications.

The platform previews Microsoft started delivering in the lead-up to Internet Explorer 9 are bare-bones browser engines on which developers can try out their site code. Platform Preview 1 of IE10 was conference in April. Ryan Gavin with Microsoft's IE team told PCMag today that "we continue to get a lot of good feedback from developers on the balance that we're striking between openness and transparency, while respecting developers' time as we deliver meaningful updates with the Platform Previews." This stands in contrast to some open source browsers' "nightly release" strategy.

Web browsers quest for HTML5 is awash in contradictions, including the fact that while it's a specification designed to foster cross-browser compatibility, the disparity among the browsers that support it couldn't be greater. Every browser implements its own subset of the standard and many times, a so-called HTML5 page will only display correctly in one. And while efforts like HTML5Test.com try to measure HTML5 support with a score, it's not so simple. Sure, a browser may respond to the most HTML5 code calls, but that says nothing about whether or not the feature was implemented correctly.

This is a major point that the IE team likes to make, especially in light of the fact that it gets a lower score on HTML5Test.com than other current browser versions. Indeed, the Microsoft team has worked intensively with the W3C, the Internet's governing standards body, submitting well over 6,000 test cases. With this second Platform Preview, IE does indeed bring some firsts in HTML5 support, though many of the updates are for features that other browsers made available long ago.

Among the catch-up HTML5 features included in today's preview: Drag-and-drop, File Reader, Forms, and Web workers. The last provides a way for Web applications to run processes in the background, thereby improving perceived performance. Among new HTML5 support on which IE10 is leading: Channel Messaging, which makes Web workers more efficient; and Page visibility, which tells a site developer whether the current page is visiblenot on a background tab or minimized. With this last tool, a developer knows whether or not to spend processing energy on the page, saving the end user not only performance drags on other running software, but saving his battery as well.

A new, unique, security feature in IE10 PP2 is HTML5 Sandbox and iframe isolation. This lets a Web developer contain elements of his site, preventing it from doing damage. For example, if a site uses a third-party ad service, the developer can use the Sandbox feature to protect the user from possible malware in an ad.

As usual, Microsoft has not only made the new browser engine available, but has published several new demos showing off its new capabilities. These include a How Stuff Works demo, which dramatically and visually shows how differently current browsers display HTML5 Canvas compositions, a Fireflies speed demo to show off GPU acceleration of HTML5 content, and Bellagio fountains, which shows the effects of Message Channel support.

You can run any of these visually striking demos by browsing to IETestdrive.com, where you can also download Internet Explorer Platform Preview 2. See below for a video showing some of what's new in the future browser.

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