WebTV offers a simulator for their browser on their site [developer.webtv.net]. There is also an easy to use online viewer at AnyBrowser.com [anybrowser.com], but it's version 1.1 of the WebTV browser, which is now up to version 2.5.
The main restriction on the WebTV browser is the screen width (550 pixels). WebTV has an algorithm that squeezes everything down to that width, no matter how you code your HTML. One big technical difference between a TV picture and a monitor is that a TV is interlaced -- only half the lines of a screen are displayed on each pass, and they alternate.
Here's 3 basic rules I follow when WebTV matters to a client.
1. Small fonts are not going to work. The WebTV browser will automatically fatten up your <font> tags, but beware of any text in graphics. Make all the letter forms substantial or they won't be legible on a TV screen.
2. Avoid all-white backgrounds -- they make a lot of TVs freak out. You'll notice that the WebTV website has sort of a light tan background.
3. Avoid intense colors like bright red -- looks like chocolate pudding, at best.
If you get into fancy stuff -- multi media and all that -- then you need to study the developer's materials that WebTV provides.
Does anyone here get significant WebTV traffic? I have two clients who felt their products warranted it, and they paid extra to develop a WebTV compatible site -- but the traffic/sales just aren't there for them.