As Neowin points out, the latest rumors suggest Spartan will be the new browser for Windows 10 mobile. One of the new "killer features" for Spartan will reportedly be an inking capability that allows you to annotate pages as you visit them, as well as share them with someone you know. This feature will work via Microsoft's OneDrive, which will store a copy of the annotated web pages so you can access it on any browser across major platforms.
Spartan also promises new ways to group tabs and may have custom skin support.
Microsoft’s new browser is said to be developed as a Windows Store app, capable of automatic updates, and will feature the same user interface across all device form factors. Internet Explorer will also be present as a legacy option, but Spartan will be the main browser moving forward.Based on some blurry photos, Neowin created a replica of what the current version of Spartan looks like. Some more details about the UI can be read over here.