Microsoft browser, Internet Explorer used a rendering engine called Trident, lived inside HSHTML.dll. The rendering engine is completely rewritten for Spartan and it is called, EdgeHTML that lives inside the EdgeHTML.dll library. The new engine is rewritten by keep in mind todays and future needs of Web developers and users.
In an article published by Jacob Rossi, Program Manager for Windows and IE at the browser team talks about the new browser called Spartan and the rendering engine. You can find more details on Smashing Magazine here >