The latest Opera browser preview version may not be entirely stable, but it's definitely got its jetpack strapped on. Opera 10.5 pre-alpha, for Windows and Mac, is the first browser that's not powered by Webkit to approach JavaScript rendering speeds previously reached only by Chrome and Safari.

Opera 10.5 pre-alpha introduces Windows 7 support and a slight redesign, along with a rocketing new JavaScript engine. (Credit: Screenshot by Seth Rosenblatt/CNET)

In empirical testing done on an HP desktop running an Intel Core 2 Q6600 at 2.66GHz with 4GB of RAM and Windows 7 32-bit, the pre-alpha scored 435.6 milliseconds in the SunSpider JavaScript benchmark. By contrast, Google Chrome, the most recent development build, notched 510.4 ms. The current stable build of Opera was more than 7.5 times slower, at 3284.4 ms.

Opera attributes this dramatic improvement to the new Carakan JavaScript engine, which they have designed from scratch to replace the Futhark engine in the stable build. Opera 10.5 also includes improvements to the Presto layout engine, and a new graphics library called Vega.

Other improvements noticeable in the pre-alpha include changing the URL address bar to include the same style of predictive smart search that Firefox and Chrome have, and the search and address bars now both remember searches, support deleting specific items, and have redesigned layouts.

The main browser interface has been redone, too. The tabs are now on top, the menu bar has been minimized behind a drop-down on the left nav, and the browser has better integration with Windows 7 and Snow Leopard. On Windows 7 there's Aero Peek and Jump List support to access Speed Dial and tabs from the Taskbar. For Macs, there's a unified toolbar, native buttons and scrollbars, multitouch gestures, and Growl support. Dialog boxes are now non-modal, which you means you can now switch tabs without a pop-up commanding your browser's focus, for example. This will affect verification and authentication pop-ups, and JavaScript alerts.

There are some known problems, including a lack of printer support in the Mac version and noticeably high memory usage. Users can expect these to get addressed before the stable build of Opera 10.50 is released. The Opera 10.50 official announcement and changes can be read here, while the current stable version of Opera 10.10 is for Windows, Mac, and Linux.