http://elm-chan.org/fsw/ff/doc/dioctl.html describes the expected ioctls to implement and their behavior. The mandatory ioctls seem pretty straightforward. CTRL_SYNC will be a nop on a browser-based storage, GET_BLOCK_SIZE should return 1 (unknown), and CTRL_TRIM looks like it could be nop (or you could zero the range to more closely match real devices).