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.
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.
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.