Can't open localhost in Microsoft Edge (Project Spartan) in Windows 10 preview

So the issue is Spartan Edge doesn't have access to the loopback addresses, which is something that most Windows Store apps are blocked from accessing.

If you are using Windows 10 RTM or build 10166, this can be done by navigating to about:flags and checking "Allow localhost loopback":

_{^{Image courtesy of Ryan Joy and used with consent.}}

If you are using a previous build of Windows 10, the CheckNetIsolation tool can be used to add it to the LoopbackExempt list. Use this from an elevated command prompt:

CheckNetIsolation LoopbackExempt -a -n="Microsoft.MicrosoftEdge_8wekyb3d8bbwe"

Microsoft.MicrosoftEdge_8wekyb3d8bbwe is the identifier for the Edge app. It can be determined by using Get-AppxPackage in PowerShell and sifting through the results for the Spartan / Edge and using the PackageFamilyName.

As of Build 10158, the PackageFamilyName has changed from Microsoft.Windows.Spartan_cw5n1h2txyewy to Microsoft.MicrosoftEdge_8wekyb3d8bbwe.

Microsoft Edge runs with network isolation by default for security reasons. To enable loopback and debug your localhost server run the following command from an elevated shell if you have 10158 or newer:

CheckNetIsolation LoopbackExempt -a -n=Microsoft.MicrosoftEdge_8wekyb3d8bbwe

or

checknetisolation loopbackexempt -a -p=S-1-15-2-3624051433-2125758914-1423191267-1740899205-1073925389-3782572162-737981194

or this one if you have an older version

CheckNetIsolation LoopbackExempt -a -n=Microsoft.Windows.Spartan_cw5n1h2txyewy

In a future update, you’ll be able to enable localhost loopback support using about:flags.

For the latest build (10158), the following command did it for me:

CheckNetIsolation LoopbackExempt -a -n=Microsoft.MicrosoftEdge_8wekyb3d8bbwe

The trick is to use the Get-AppxPackage PowerShell command to find out the family name of Edge app as vcsjones mentioned.