Clarify core media types

[mattgarrish]

Picking up on #1341 (comment), this is my best attempt to expand on the perennially confusing nature of core media types.

Let me know if there's anything you don't agree with, or if any parts need further elaboration.

Feedback is always welcome beyond the reviewer list, too.

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[TzviyaSiegman]

This looks clear to me. The only minor change I might suggest is s/compact/contract.

[murata2makoto]

Since CSS is a core media type, I tried to check when the support of CSS is required.

Neither 2. Conformance Requirements nor
4.1.1 General Conformance mentions the support of CSS.

I am not mistaken, 4.3 CSS Style Sheets mandates the support of CSS even when the RS does not have a viewport.

[dauwhe]

I am not mistaken, 4.3 CSS Style Sheets mandates the support of CSS even when the RS does not have a viewport.

Section 2 covers this:

If it has a Viewport, it MUST support visual rendering of XHTML Content Documents as defined in § 4.3.1 General Conformance.

[murata2makoto]

@dauwhe

You are right. But I think that the sentence you quoted should mention CSS.

[iherman]

(This is not part of the changed, ie, I could not comment directly...)

The section on foreign resources say:

When a Foreign Resource is included in the spine or directly rendered in its native format in an EPUB Content Document, a fallback Core Media Type Resource MUST be included. Fallbacks take one of the two following forms:

• intrinsic fallback mechanisms provided by the format (e.g., ability to provide more than one media type or to display an alternate embedded message when a media type cannot be rendered);

• manifest fallbacks.

I am not sure I properly parse the first bullet items. I presume we refer to the fact that, e.g., the HTML <picture> element allows me to add several formats and let the engine choose the best. But the term "format" in that sentence seems to refer to the media element (e.g., the picture in a format that cannot be used by the RS) and not to the content document...

[mattgarrish]

But I think that the sentence you quoted should mention CSS.

Agree, but I'll open a separate PR for this so we don't mix issues.

[mattgarrish]

But the term "format" in that sentence seems to refer to the media element

Ya, it refers to the embedding format - SVG or XHTML or whatever the foreign resource is included in.

[mattgarrish]

But the term "format" in that sentence seems to refer to the media element

I've changed this to:

intrinsic fallback mechanisms provided by the host format (e.g., [HTML] elements often provide ...