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Computerworld - Microsoft on Wednesday updated its bare-bones preview of Internet Explorer 9 (IE9) for the last time, saying that the next release would be a beta build.

Although Microsoft hasn't named a release date for IE9's beta, the six-to-eight week stretch between each Platform Preview may provide a clue: If the company sticks to the same gap between the fourth preview and the beta, the latter should show on or after Sept. 15.

In IE9 Platform Preview 4, Microsoft has integrated its new JavaScript engine into the browser, finished its work on hardware acceleration and boosted performance in several areas, including the Acid3 test, said the IE team's leader.

"The IE9 platform is nearly complete," said Dean Hachamovitch, general manager of IE, in a detailed post on the browser's blog Wednesday.

Last updated six weeks ago, the IE9 Platform Preview is not a full-fledged browser, but instead consists of a minimalist interface wrapped around Microsoft's newest rendering and JavaScript engines.

When Microsoft debuted IE9 in mid-March, the company committed to updating the IE9 preview approximately every eight weeks until it issues a public beta. That beta is slated to ship next month, according to comments made last week by Kevin Turner, Microsoft's chief operating officer.

The fourth preview includes fixes for previously reported bugs, wraps up the graphics processor-powered audio, video and text acceleration, and moves Microsoft's new "Chakra" JavaScript engine inside the browser for better performance, Hachamovitch said.

"One aspect of [meeting our goals] is integrating the JavaScript engine natively inside the browser, rather than bolting it onto the side to support multiple JavaScript engines as some other browsers do today," Hachamovitch said, taking a shot at rivals such as Mozilla's Firefox. "How a JavaScript engine is integrated into the browser is as important as the engine itself for real-world HTML5."

Although Microsoft once downplayed JavaScript speeds -- Hachamovitch dismissed the contest for fastest browser as just a "drag race" during IE8's development -- it's aggressively touted Chakra's performance.

According to Hachamovitch, IE9 Platform Preview 4 scores better on the SunSpider JavaScript test suite than all rivals except Google's Chrome and Opera Software's Opera. Microsoft's SunSpider results differ from Computerworld's latest, which pegged Apple's Safari as the fastest on Windows, with Opera and Chrome close behind.

On the Acid3 benchmark, which checks how closely a browser follows certain Web standards, the IE9 Platform Preview 4 scored 95 out of a possible 100, the highest ever for a Microsoft browser. The score was a 14% improvement over Platform Preview 3.

Hachamovitch hinted that the score of 95 would be IE9's best effort, arguing that the standards which prevent it from acing the benchmark are "in transition" and thus unlikely to get full support from Microsoft.