HTTP Strict Transport Security (or HSTS) is an important aspect in web security. IdentityServer3 provides a configuration option to include the HSTS headers in all of its HTTP responses. To enable, use the UseHsts extension method on the IAppBuilder in your OWIN configuration:

public void Configuration(IAppBuilder app)

    // ...

If you wish to set the expiration (max-age), then UseHsts has overloads that accept an int for the number of days, or a TimeSpan for a custom duration. A value of 0 or TimeSpan.Zero can be used to purge the HSTS browser cache. The default expiration is 30 days.