In order to debug a website served on localhost by IIS Express you need to use localhost.fiddler to get traffic to show up in Fiddler, but if you use HTTPS you'll first get a warning that the certificate name does not match and you have to manually confirm this connection. This is generally a good idea, but in the case of localhost and localhost.fiddler I think this warning should not be shown, as having to use localhost.fiddler is simply a technical restriction that Fiddler should know about...