This is a more enhanced version of the WebTV box. It may well work on analogue but is really intended for deployment in a digital TV environment.
The triggering and standard HTML support is defined according to the ATVEF platform specifications which are evolving and are not yet complete but are looking to be the definitive profile that 'Browser in a box' type of system needs to conform to.
Technically, this platform is more advanced than WebTV but it is fundamentally the same.
Detecting that you are running in a set top box may not always be easy. The user agent values may not tell you that you have a DTV environment available and you may need to be a little more clever and test for the existence of specific object classes. You need to do that in a way that does not cause a run-time error of course. Its a good technique to practice to ensure your scripts run reliably regardless of whether they are designed for use with Microsoft TV or not.