In Internet Explorer 3.0, a number of the properties supported by Navigator 2.0 and 3.0 are "supported" only in the sense that they can be used without causing errors. These properties may not return meaningful values when read and/or do not cause any changes when set. Some properties, like Document.alinkColor are non-functional simply because the browser as a whole does not support the feature (special colors for activated links, in this case). Others are simply not supported presumably because the engineers at Microsoft did not have the time to implement them. These include the action, encoding, method and target properties of the Form object and the length property of the History object.
The isNaN() function also falls into the category of "supported but nonfunctional." Because IE 3.0 does not support NaN values, the isNaN() function always returns false.