Intent to ship: Render unknown MathML elements as an mrow

[Eri Pazos Pérez]

We intend to turn “Render unknown MathML elements as an mrow” on by default. A combined intent to prototype and ship follows.

Summary: The MathML Core specification indicates that “Unknown MathML elements behave the same as the mrow element”. This means any element not explicitly defined within the specification should be rendered as an mrow.

Example: In general, the use case is to allow web developers to write polyfills for MathML3 (e.g. <mfenced>) or for future elements (e.g. <a>).

In this example there are clickable links within mathematical expressions. With this change, the <a> elements act as an mrow since they are unknown to the MathML Core specification, ensuring the fraction displays correctly.

<math display="block">
  <a href="https://en.wikipedia.org/wiki/Momentum">
    <msub>
      <mi>p</mi>
      <mi>k</mi>
    </msub>
  </a>
  <mo>=</mo>
  <mfrac>
    <a href="https://en.wikipedia.org/wiki/Lagrangian_mechanics">
      <mo>&#x2202;</mo>
      <mi>L</mi>
    </a>
    <a href="https://en.wikipedia.org/wiki/Generalized_coordinates">
      <mo>&#x2202;</mo>
      <mover>
        <msub>
          <mi>q</mi>
          <mi>k</mi>
        </msub>
        <mo >&#x02D9;</mo>
      </mover>
    </a>
  </mfrac>
</math>

Before:

After:

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1958853.

Specification: MathML Core, https://w3c.github.io/mathml-core/#ref-for-dfn-unknown-mathml-element-2.

Standards Body: W3C, relevant discussion at https://github.com/w3c/mathml-core/issues/71.

Platform coverage: All platforms.

Preference: mathml.unknown-mrow.

Other browsers:

• Blink: shipped in https://chromium-review.googlesource.com/c/chromium/src/+/2007738.
• WebKit: shipped in https://bugs.webkit.org/show_bug.cgi?id=123348.

web-platform-tests:

• Added new test cases for mfenced and a in:
  • mathml/presentation-markup/mrow/dynamic-mrow-like-001.html
  • mathml/presentation-markup/mrow/inferred-mrow-baseline.html
  • mathml/presentation-markup/mrow/inferred-mrow-stretchy.html
  • mathml/presentation-markup/mrow/mrow-preferred-width.html
  • mathml/presentation-markup/mrow/spacing.html
  • mathml/presentation-markup/operators/embellished-operator-001.html
• There are legacy elements from the MathML3 spec that are not treated as unknown for backwards compatibility, including mlabeledtr and menclose. The latter already has tests, but a new internal one was added for mlabeledtr:
  • mathml/tables/mtable-mlabeledtr-ref.html

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