This value will be set to true if the window contents were delivered from a secure source (probably via the https: protocol). Normally this value will be false, as is the case if the window is loaded from a local file. That would seem a little strange in that you probably have to have local access control permissions to reach a file on the local file system and one would have thought that would be secure and meriting a true value in this property.

Property attributes:

ReadOnly.