Defaults for font families

I was comparing a few browser's actions with generic font families, and I
found some interesting results.

Basically, IE used appropriate fonts all around. NN used a sans-serif font
for cursive and a music notation font I have installed (Power Tab) as
fantasy. Opera used a serif font for cursive.

How do these browsers actually select the default fonts? Surely Netscape
isn't programmed to choose Power Tab as a fantasy font?

And are these defaults actually useful, especially cursive and fantasy,
considering that on these three browsers, on my one computer, the fonts
don't even remotely resemble each other, and in some cases don't even
resemble the type of font they're supposed to represent?