http://www.zdnet.com/article/microsofts-spartan-browser-whats-under-the-hood/ [zdnet.com]

Microsoft's Spartan browser: What's under the hood

Sources have said that Spartan will use the same rendering engine as IE. In Microsoft's blog post about Spartan, company officials say Spartan is "powered by a new rendering engine." That rendering engine is the same as the one in IE 11, which as far as I know, is an updated version of Microsoft's Trident. (It's definitely not WebKit, Blink or Gecko.)