I did an amount of research about the MIME-type errors I was getting.
It turns out Webkit (that means both Safari and Chrome, as well as some others) has a problem with caching, such that errors in a file will remain permanently. I flushed the cache, and it all works (except for the favicon, but I’ve never liked .ICOs).
The reason things went off was that I had defragmented my flash drive, which is incidentally also my localhost. Taking most of those files off and putting them back on fixed most of it, but some javascript files were stubborn.
I have no clue why it transmits my favicon as text/html, but I the problem with my JS was that it was interpreting it as ‘other’, even though it was being sent as javascript. Even completely replacing the file didn’t work.