Internet Explorer 10 incorporates HTML5 and CSS3 leaving appart menues and complements bars.
Also you can use the whole screen to web and browse with templates that are included . New interface and design are also introduced.
You can also see the official site of Redmond company which explains new Microsoft browser features also added on Outlook.com
Outlook.com mail is a free service which competes with Gmail and now has as a tool new Web IE 10. Better browsing speed and tech are the most important reasons of the selection of this Explorer to be part of Outlook.com