epub:type restricted to palpable content may cause backward incompatibilities

[rdeltour]

In EPUB 3.3, XHTML requirements says the epub:type attribute MUST only be used on palpable content.

This list is restrictive and will likely cause a lot of existing material being rejected. Several HTML elements like body or li are not palpable content but are often associated an epub:type attribute (they're defined as relevant usage context in the structural semantics vocabulary).

Also, it means epub:type and hidden cannot co-exist, I'm not sure how that's relevant.

Was the idea was to not allow epub:type on some elements like head and style?

If yes, we could either:

• restrict epub:type to any element except metadata content and the html element.
• restrict epub:type to flow content and the body element.

[mattgarrish]

epub:type is not supposed to be used on the head element or within it as the header doesn't contain structural content.

The restriction was previously only on reading systems to ignore the attribute in the head element, but an authoring restriction was added to match. Agree that palpable doesn't look right.

I'm fine switching it to "MUST NOT be used on the head element or on metadata content". This would also capture uses on script, link, etc. in the body, as these are not structural elements in the sense that the attribute was created for.