because they aren't words in the dictionary
How much spell checkers should be ahead of the dictionaries and language in technology terms is, I would say, and issue of contention.

However, for everyday language and non-technical terms I'd say that the dictionary compilers are a better starting point. It's not up to hackers to decide which words are in a language.

Thus `stylesheet' should perhaps be included, but composable should not. Why? Because it's not a word. It's not in any English dictionary I know of and isn't generally used. The stems are fine, but just sticking stems and words together doesn't mean a spell checker should respect them. Do they include spellcheckerable?

because they aren't words in the dictionary
True enough. However perhaps there should be a rule where it becomes a word when google returns more than 100,000 matches (composable returns 102,000)

If you get too literal on me you're going to take all the fun out of my simple attempt at witty humor.
Dave

100,000 hits on Google
Is Viagra in the dictionary yet?