I like the idea, but fonts are hard to choose from, especially with a specimen as regular as a CSS file. If I had to pick a file type with variety, I might go for PHP, with inline HTML, CSS, and Javascript, and attempt to show all the symbols on the keyboard, as well as common ligatures.

That said, there's so much variety in fonts that someone's favorite is likely missing regardless of the selection. Perhaps an initial culling based on features would help, eg. ligatures/no ligatures, serif/sans-serif, etc.

I'm also not really able to firmly decide, so I tend to switch fonts every so often.