There's been talk for a while that Microsoft was going to make some big changes to Internet Explorer in the Windows 10 time frame, making IE "Spartan" look and feel more like Chrome and Firefox.

It turns out that what's actually happening is Microsoft is building a new browser, codenamed Spartan, which is not IE 12 -- at least according to a couple of sources of mine.

Thomas Nigro, a Microsoft Student Partner lead and developer of the modern version of VLC, mentioned on Twitter earlier this month that he heard Microsoft was building a brand-new browser. Nigro said he heard talk of this during a December episode of the LiveTile podcast.

Spartan is still going to use Microsoft's Chakra JavaScript engine and Microsoft's Trident rendering engine (not WebKit), sources say. As Neowin's Brad Sams reported back in September, the coming browser will look and feel more like Chrome and Firefox and will support extensions. Sams also reported on December 29 that Microsoft has two different versions of Trident in the works, which also seemingly supports the claim that the company has two different Trident-based browsers.

However, if my sources are right, Spartan is not IE 12. Instead, Spartan is a new, light-weight browser Microsoft is building.

Windows 10 (at least the desktop version) will ship with both Spartan and IE 11, my sources say. IE 11 will be there for backward-compatibility's sake. Spartan will be available for both desktop and mobile (phone/tablet) versions of Windows 10, sources say.

Spartan is just a codename at this point. My sources don't know what Microsoft plans to call this new browser when it debuts. The IE team hinted during a Reddit Ask Me Anything earlier this year that the team had contemplated changing the name of IE to try to get users to realize the much more standards-compliant IE of today is very different from older, proprietary versions of IE.

Microsoft may show off Spartan on January 21 when the company reveals its next set of Windows 10 features. But my sources also aren't sure if Spartan will be functional enough for inclusion in the Windows 10 January Technical Preview and mobile preview builds that are expected to be available to testers in early 2015. It may not show up in the test builds until some point later, they say.

Will Microsoft end up porting the Spartan browser to Android, iOS and/or any other non-Windows operating systems? I'm not sure. The IE team said a few months back that Microsoft had no plans to port IE to any non-Windows operating systems. But Spartan isn't IE. And these days, Microsoft is porting much of its software and services to non-Windows variants. So I'd say there's a chance that this could happen somewhere down the line ....