Use of ftyp box

[cconcolato]

The BSF spec indicates the following about the "File Type Box" (https://www.w3.org/TR/mse-byte-stream-format-isobmff/#iso-init-segments):

An ISO BMFF initialization segment is defined in this specification as a single File Type Box (ftyp) followed by a single Movie Box (moov).
The user agent must run the append error algorithm if any of the following conditions are met:

1. A File Type Box contains a major_brand or compatible_brand that the user agent does not support.
2. A box or field in the Movie Box is encountered that violates the requirements mandated by the major_brand or one of the compatible_brands in the File Type Box.

In my testing, none of the browsers I tested (Chrome, Firefox, Safari) care about the presence or contents of the ftyp box.
Bullet 2. above would mean that browsers have a mapping (brand, requirements), which I don't think they have.
Also 1) contradicts the design of ISOBMFF which says that you can process a file if you understand at least one brand.

I think the BSF can safely be updated to match what implementations do and simply say that an Initialization Segment is a moov box, optionally preceded by boxes.

Note that the same type of sentence exists also for the styp box, and could be safely removed I think.

[wolenetz]

From Sep 28 Media WG teleconference (https://www.w3.org/2021/09/28-mediawg-minutes.html), we had agreement that updating the BSF to match what implementations do (not require ftyp, and if it's syntactically correct in the stream - e.g. in the right parsing position in the grammar - skip it without checking brands or UA support for them.) With such skipping, the other references in the BSF to ftyp's brands and their requirements should also be dropped. The same goes for styp.

We could also assist content providers and implementors by providing a non-normative note describing this -- essentially, implementations should attempt to parse and play what they can, and that ftyp and styp brands' support verification has not been implemented and enforcing such verification could regress existing content playback.