Detecting Windows Phone and Tablet clients This is called from a VF page, 

styleClass="{!IF(isWindows8Device, 'windows', 'not_windows')}" 


So if it's a windows device, return true.

The best thing to do would be to add one method in DeviceDetectionCls to check for any windows device, instead of two methods.

For the purpose of this question though, I've to use both.

Could it be improved at all? 

public Boolean isWindows8Device{    
    get{
         String userAgent = System.currentPageReference().getHeaders().get('User-Agent');                       

         if(DeviceDetectionCls.isWindows8Phone(userAgent)){
            return true;
         }

         if(DeviceDetectionCls.isWindows8Tablet(userAgent)){
            return true;
         }

         return false;
    }
}
I guess something like :

return DeviceDetectionCls.isWindows8Phone(userAgent) || DeviceDetectionCls.isWindows8Tablet(userAgent);


would do the trick.