[FEATURE] HTML hyperlinks in MathML

[TimArborealis]

Problem Statement

We currently have no instructions on nesting HTML hyperlinks in MathML markup.

Proposed Solution

Embedded HTML hyperlinks inside a MathML expression.

Alternative Solutions

Presenting links as plain text in an <mtext> element.

Use Cases

Source citations for equations or links to further reading.

Impact on Suppliers

Unsure.

Implementation Considerations

It is unclear whether this is possible within MathML 3, which would cause problems in the current Webarch validator. It should be possible according to MathML Core 2.2.1 HTML and SVG, but reading system support might be poor and would have to be tested thoroughly.

Examples

From Sami's test book:

<math xmlns="http://www.w3.org/1998/Math/MathML">
        <mo>|</mo>
        <mi>x</mi>
        <mo>+</mo>
        <mi>y</mi>
        <mo>|</mo>
        <mo>&lt;</mo>
        <mo>|</mo>
        <mi>x</mi>
        <mo>|</mo>
        <mo>+</mo>
        <mo>|</mo>
        <mi>y</mi>
        <mo>|</mo>
        <sup>
          <mtext>
            <a href="https://en.wikipedia.org/wiki/Triangle_inequality">Triangle inequality (Wikipedia)</a>
          </mtext>
        </sup>
      </math>

Although this yields "Math input error" in Calibre, so another method might be needed. Also <msup> should probably be used instead of <sup>.

Additional Context

This is what the MathML 3 spec says:

3.2.2.2 Embedding HTML in MathML
MathML can be combined with other formats as described in Section 6.4 Combining MathML and Other Formats. The recommendation is to embed other formats in MathML by extending the MathML schema to allow additional elements to be children of the mtext element or other leaf elements as appropriate to the role they serve in the expression (see Section 3.2.6.4 Mixing text and mathematics).

[samimaattaCelia]

<msup> introduces problems in MathCAT, since it will say "to the power of hyperlink ...". This is why I think semantically it would make more sense to use <sup>. It is also used in regular HTML and EPUB markup to mark references.

Of course, we could direct Neil to make exceptions for this kind of markup to treat it as text:

<msup>
  <mtext>
      ...
   </mtext>
</msup>

The "Math input error" comes form MathJax, which doesn't handle MathML and HTML mixed markup (I think at all).

[MartheGjelstad]

Sami will draft a possible solution. The suggestion is to do it the same way as with footnotes.