Does the WebM specification clearly prohibit video/webm;codecs=h264 and video/webm;codecs=avc1 even when the media is ultimately encoded as a Matroska container?

[guest...@gmail.com]

Does the WebM specification clearly prohibit

video/webm;codecs=h264

and

video/webm;codecs=avc1

even when the media is ultimately encoded as a Matroska container?

Specifcally MUST is not contained in the language at https://www.webmproject.org/about/

WebM files consist of video streams compressed with the VP8 or VP9 video codecs and audio streams compressed with the Vorbis or Opus audio codecs.

For example, at Chromium 77

MediaRecorder.isTypeSupported("video/webm;codecs=h264")

 

and 

MediaRecorder.isTypeSupported("video/webm;codecs=h264")

each return 

true

while

mediainfo

outputs 

General
Complete name                            : h264.webm
Format                                   : Matroska
Format version                           : Version 4 / Version 2
File size                                : 3.13 MiB
Writing application                      : Chrome
Writing library                          : Chrome
IsTruncated                              : Yes
FileExtension_Invalid                    : mkv mk3d mka mks


Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Codec ID                                 : V_MPEG4/ISO/AVC
Width                                    : 500 pixels
Height                                   : 500 pixels
Display aspect ratio                     : 1.000
Frame rate mode                          : Variable
Language                                 : English
Default                                  : Yes
Forced                                   : No

[James Zern]

Hi,

On Tue, Jul 2, 2019 at 6:54 PM <guest...@gmail.com> wrote:
>
> Does the WebM specification clearly prohibit
> video/webm;codecs=h264
>
> and
> video/webm;codecs=avc1
>
> even when the media is ultimately encoded as a Matroska container?
>
> Specifcally MUST is not contained in the language at https://www.webmproject.org/about/
>

The codecs are marked as SHOULD [1]. Parsers may handle the format
differently, but it can always be parsed as an mkv file.

[1] https://www.webmproject.org/docs/container/