"valid MIME type" is the wrong primitive

[annevk]

This specification uses "valid MIME type", but instead wants to use the MIME type parser, also defined in the MIME Sniffing standard. Validity is not a concept that user agents typically enforce.

[chrisn]

This was updated in 95f037c, @annevk, does the latest draft look OK to you? Thanks.

[annevk]

Some nits:

• Failure should not be formatted as code.
• What does "unrecognized" mean?
• "container type" is not a thing. https://mimesniff.spec.whatwg.org/#mime-type defines the MIME type concept and its member concepts (which ideally you link).
• You don't have to talk about case-sensitivity of MIME type's type comparisons as type is lowercased after parsing and any list of supported types should be too.
• Are there still user agents that do not recognize either "codecs" or "profiles"? (Arguably accessing these parameters should be done using Infra's map notation.)