Microsoft has unveiled the fourth preview of its upcoming web browser, Internet Explorer 10, which is now available for developers to download. The browser appears to be evolving into an application platform for HTML5, with nearly all of its improvements revolving around the updated markup language to help developers build Web-optimized applications.

These foundational capabilities are what developers building native applications depend on: working with binary data and files, controlling selection and hit testing in application UI, and providing accessible video content with captioning, said Rob Mauceri, program designer of the IE group at Microsoft.

Microsoft showed off the IE10 preview on the Windows 8 developer preview. Here are the best features of the new platform.

More HTML5 support: More HTML5 support means richer Web apps with improved performance. IE10 is apparently behind Mozilla Firefox and Google Chrome when it comes to overall HTML5 feature support, but Microsoft's browser has surpassed Safari and Opera, which originally proposed HTML5. IE10 reportedly supports 83 percent of HTML5 recommended, proposed, and working draft features. Firefox and Chrome support 86 and 89 percent of HTML5 features, respectively, while Safari and Opera support only 77 percent and 70 percent, respectively.

Video captioning: In IE10, users can select any part of a playing video and add their own captions. Users can add multiple captions to any video, and IE10 will synchronize the captions with the content. IE10 also allows users to be able to search the captions if they want to jump to a specific point in the video. Microsoft says IE10 is the first platform that supports track captioning and TTML and WebVTT formats, and give developers the option to choose which format works best for their needs.

Fast rendering: Internet Explorer 10 will support a steady 60 frames per second, despite dozens of visual elements occurring simultaneously, including gradients, opacity and touch interaction. Other platforms, like Google Chrome, have a tough time rendering so many elements, and usually average about three or four frames per second.

CORS support: Internet Explorer 10 will support cross-origin resource sharing (CORS), which helps users and developers safely share data and information across domains using XML and HTTP requests. Developers can drag and drop their files into an easy-to-use uploading platform in order to build and update their websites safely and securely. The platform can upload multiple files at once, and run the Web app in one domain and upload the files to another domain for storage. A visual feedback circle indicates the progress of the uploads.

New CSS support: A new function called CSS user-select allows developers to control which parts of their app can be selected by the user. The entire site can be selectable or only a few specific areas, such as blog posts or commenting areas. This helps developers keep application UI separate from the content.

Mauceri says the features in the latest platform preview are available to Web pages now and will soon be available to the Metro-style applications in Windows 8.

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