MP4-files should be seekable

Currently, it appears that browsers that use MP4 files (mostly: Safari, on iOS and MacOSX) with JWplayer read without being able to seek-forward.

Seek-forward means that it is possible for the user to click further forward and that the player does not need to wait till the download of the whole video till there.

This feature is enabled in WebM by the use of an index which allows the Apache-supported Range header to download parts of the file expressed in bytes. It seems that some UAs express ranges in seconds, which requires a deep understanding of the MP4 file by the server and restricts the possible encodings.