The current MSE draft returns an error if a Movie Extends (mvex) box is contained in the Movie Header Box (mvhd), but doesn't explicitly state that an mvex is required.  There are two issues with this:

1.  MSE should require an mvex box to indicate fragmented content and doesn't now.  An mvex is not required in the base format documents, so it makes sense for MSE to spec this.
2.  The base format specs don't allow an mvex in the Movie Header Box, so it's not necessary for this to be explicitly listed in the MSE spec.

Changing the spec to require an mvex in the moov can address both issues.

Detail is to change the error condition in line 2 of 12.2.1 from:

2.  A Movie Extends (mvex) box is contained in the Movie Header Box to indicate that Movie Fragments are to be expected.

To:

2.  A Movie Extends (mvex) box is not contained in the Movie (moov) box to indicate that Movie Fragments are to be expected.

Section link:  https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html#iso-init-segments