Ambiguous exemption for "embedded" resources

[bduga]

In section 3.4 "Exempt resources" we have the following bullet point for a blanket exemption:

"it is not embedded directly in EPUB content documents (e.g., via [html] embedded content and [svg] image and foreignObject elements)."

It is unclear what is meant by embedded as we never define the term. Valid interpretations of the term would make scripts, style sheets, and/or fonts exempt (or not). We should clarify what the term means, or just better explain the types of exempt resources.

See discussion in #2655 for background.