Update Content MathML semantics requirements

[bentley]

The manual states: “When possible, Content MathML is used over Presentational MathML.”

This contradicts the EPUB spec:

The math element MUST contain only Presentation MathML, with the exception of the annotation-xml element.

Content MathML MAY be included within MathML markup in XHTML Content Documents, and, when present, MUST occur within an annotation-xml child element of a semantics element.

So I modified the example, basing it on MDN and the MathML spec.

[bentley]

I looked at all the MathML in the corpus and didn’t see any Content MathML, so this shouldn’t require immediate change in any existing books.

[acabal]

Thanks!