ROMFS is used to structure HTML, media, and other files that will later be read by WebTV/MSN TV clients. So far, it's known that dedicated firmware ROMs, Viewers, and WebTV for Dreamcast utilize this format to store system files. Not much information is available on how it's handled on official WebTV/MSN TV implementations at the moment. So far, ROMFS has only been documented in code in both a collection of ROMFS libraries and a now-deleted build info tool, both written by eMac (the former being published by us with his permission). The ROMFS libraries also now come with a couple sample scripts written by us that you can use to extract and rebuild ROMFS data with ease.
Formal documentation on this format will be added here to the best of our abilities when we get time.