Along with the announcements about new features and improvements included in IE of the preview build of Windows 10, IE team also announced the new Experimental features section. This new experimental features dashboard can be accessed by browsing to about:flags where you will find new platform experiments that you can enable to try out the very bleeding edge of what Microsoft is working on.
Given the sheer volume of changes in Edge mode, we’re going to progressively roll out the new mode by choosing a random set of Windows Insiders to get Edge while the rest remain in 11 document mode. Our data science and quality engineering teams will leverage user feedback and anonymous telemetry to guide the roll out while we make tweaks to the platform in future preview builds accordingly.
If you’re a developer coding against new features or testing for compatibility, you can head over to about:flags and choose Edge mode explicitly by setting “Enable Experimental Web Platform Features” to “Enabled” (“Automatic” restores you to the default roll out and “Disabled” lets you compare with 11 mode).
Source: IE Blogs