Microsoft has released a new Internet Explorer 10 preview, the second pre-release of Internet Explorer 10 designed to give developers access to the new technologies that Internet Explorer 10 will deliver. The new version includes support for a bunch of new specifications, enabling better support for drag and drop, form validation, positioning of page elements, and more. As with prior preview releases, Microsoft has also provided a number of demo sites to show off new capabilities, and new test cases to demonstrate exact conformance with the HTML5 specifications.

This update comes 11 weeks after the first preview release, making it a little ahead of schedule; Internet Explorer 9 previews came out roughly every eight weeks, but for Internet Explorer 10, Microsoft is aiming at one every three months.

Probably the most significant inclusion for Web developers is support for the Web Worker API. Conventionally, browser JavaScript has been strictly single-threaded, with no facility to perform computations in the background or use multicore processors. Web Workers change this, by allowing multiple scripts to execute simultaneously, opening the door to pages that are both more responsive and contain more complex scripting than is possible without Web Workers.

Throughout the development of both Internet Explorer 9 and Internet Explorer 10, Microsoft has shied away from implementation specifications that are still in flux or undergoing extensive modification, sticking instead to those that are reasonably stable and thoroughly debugged. The company's intent has been to ensure that those features it implements won't be a maintenance liability for developers—that the features can be used and deployed onto real sites, without requiring extensive busy-work to track specification changes.

Internet Explorer 9's previews did diverge from the goal slightly, with the introduction of performance tracking mechanisms that Microsoft implemented not long after their specification was first proposed. With Internet Explorer 10, the company is doing a similar thing on a slightly larger scale, with a feature named Positioned Floats. Developed in conjunction with Adobe, Positioned Floats enable powerful page layouts not readily attainable with current CSS. The new design capabilities afforded by Positioned Floats should excite developers, but they can't become standardized unless other browsers also implement them. Hopefully, others will follow Microsoft's lead.

Preview 2 also includes support for the new Web File API, which gives browsers controlled, restricted access to the local file system. The company has been experimenting with this since early May, when it shipped a prototype implementation as part of its HTML5 Labs project. This is the first time that a technology has gone from HTML5 Labs into Internet Explorer proper. Though the prototypes are much less widely-used than the platform previews (which typically weigh in at about a million downloads), Microsoft says that the experience it gained developing the prototypes gave valuable insight into the way the API should work.

The breadth of new features added to Preview 2 give Microsoft's browser a substantial boost on the HTML5 Test site, scoring 231 points, compared to Preview 1's 125, and Internet Explorer 9's 141 (yes, the older browser scores better, for some reason). Though the selection of features tested on that site is quite arbitrary—98 points are available for new HTML5 form capabilities, but just 15 for the far more significant Web Workers feature, for example—and the testing isn't very thorough (for the most part, it doesn't test that features actually work correctly), this still represents a healthy boost, and shows that Internet Explorer 10 is making great progress in implementing these new features, just as its predecessor has done. Internet Explorer still lags the competition a little (Firefox 5 scores 296, Chrome 12 gets 327), but the gap is certainly closing.

The new version scores very highly in a new JavaScript test suite called test262. Still under active development, test262 contains more than 10,000 tests, with a goal of fully covering the entire ECMAScript specification—better known as JavaScript—to prove that implementations conform with the entire language. Platform Preview 2 currently scores 10862 out of 10865. Firefox 5, by way of comparison, passes only 10660 tests. Microsoft regards the performance in this kind of thorough test as being rather more important than the performance in scattershot suites such as HTML5 Test and Acid3. A pass in HTML5 Test or Acid3 doesn't necessarily indicate that a feature has been fully and robustly implemented, nor that it has been implemented exactly as the specification says. To do that requires detailed conformance tests. Though these are much harder to produce than the smaller tests, and, especially when compared with Acid3, are much less visually appealing, they give a much better indication of whether a feature really has been implemented properly.

Internet Explorer 10 shows that Microsoft is continuing to invest heavily in bringing Internet Explorer up to the same standard as its competition—and sometimes even beyond. There are a few more big ticket items that would make developers happy—Web Sockets, currently still a Labs prototype, is particularly desirable—but those seem well within reach. Microsoft isn't willing to talk about release dates at present, though is widely expected to ship Internet Explorer 10 some time around next March. With the way the browser is shaping up so far, it should be a very strong contender indeed.