"thin", "thick", "medium" values mfrac@linethickness

[fred-wang]

From https://mathml-refresh.github.io/mathml/chapter3.html#presm.mfrac :

Specifies the thickness of the horizontal "fraction bar", or "rule" The default value is "medium", "thin" is thinner, but visible, "thick" is thicker; the exact thickness of these is left up to the rendering agent.

The MathML Core spec suggests 50%, 100%, 200%.

It's possible that we also want to remove/deprecate these values if they are not used in practice.

Math WG discussion: https://lists.w3.org/Archives/Public/www-math/2016Aug/0001.html

[physikerwelt]

I think we should remove the string constants "thin" (50%), "thick" (200%), "medium" (100%)

[fred-wang]

cc @rwlbuis, @emilio :

This was removed from MathML core in w3c/mathml-core@ce02fd1

I'm leaving this open because it still remains to:

• Remove WPT tests if any.
• Send intent-to-remove to mozilla/webkit-dev mailing lists (maybe old Math WG too?).
• Remove browser implementation.
• Decide what to do for the full MathML spec (remove values? deprecate them? specify explicit numbers?).
• Add polyfill to https://github.com/mathml-refresh/mathml-polyfills (e.g. convert to linethickness="50%", "100%", "200%" respectively).

[fred-wang]

Decision is to keep in MathML full but note in main spec that non core features will require polyfills so you may not want to use them. https://lists.w3.org/Archives/Public/public-mathml4/2019Mar/0026.html

[fred-wang]

Resolution:
#4 (comment)

Specification:
Add deprecation warning in Full?

Implementation:
WebKit: https://bugs.webkit.org/show_bug.cgi?id=196142
Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1548529
Not implemented in Chromium.

Polyfill:
Convert to length values
Maybe using CSS variables to make values configurable?

Tests:

• Remove WPT tests or check that these values are not supported.

[NSoiffer]

MathPlayer uses the values 4.0, 7.5, 12.0. I'm not certain, but it appears these values are a percent of the font size. The default value is 5.0.

Here's an example from MathPlayer:

<math display='block' xmlns='http://www.w3.org/1998/Math/MathML' mathsize="24pt">
 <mrow>
  <mfrac linethickness="thin">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac>
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac linethickness="medium">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac linethickness="thick">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  </mrow>
</math>

I'm not particular fond of the MathPlayer values. Here's a modification that uses the values @fred-wang proposed:

<math display='block' xmlns='http://www.w3.org/1998/Math/MathML' mathsize="24pt">
 <mrow>
  <mfrac linethickness="0.5">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac>
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac linethickness="1">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  <mfrac linethickness="2">
   <mn>5</mn>
   <mn>888</mn>
  </mfrac>
  </mrow>
</math>

More realistically, here it is at 12pt:

Which seems fine to me. 10pt renders similarly (i.e, the line thickness is distinguishable).

[fred-wang]

@davidcarlisle As said in #4 (comment) this was removed from core. Please do the update in mathml full, if any before removing the label.

[NSoiffer]

As per my action item, MathJax uses these values: 0.67, 1, 1.67. These seem to be multiples of the rule thickness.

[fred-wang]

Firefox and WebKit use values .5, 1 and 2:
https://searchfox.org/mozilla-central/source/layout/mathml/nsMathMLmfracFrame.cpp#31
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/mathml/MathMLFractionElement.cpp#L63

[fred-wang]

It seems MathJax, Firefox and WebKit values are relatively close so I'm happy to take any of them.

It looks like MathJax does not use the factor for medium/thin and thick/medium, but that's also the case for the CSS-based proposal #7

[fred-wang]

https://lists.w3.org/Archives/Public/public-mathml4/2019Jun/0013.html

NO Objections - ADOPTED 067, 1.0, 1.67

[fred-wang]

Polyfill:

window.addEventListener("load", function() {
    const namespaceURI = "http://www.w3.org/1998/Math/MathML";
    Array.from(document.body.
               getElementsByTagNameNS(namespaceURI, "mfrac")).
        forEach(mfrac => {
            if (!mfrac.hasAttribute("linethickness"))
                return;
            var value = mfrac.getAttribute("linethickness").
                replace(/thin|medium|thick/, match => {
                    switch (match) {
                    case "thin": return "67%";
                    case "medium": return "100%";
                    case "thick": return "167%";
                    }
                });
            mfrac.setAttribute("linethickness", value);
        });
});

[fred-wang]

Tested by https://w3c-test.org/mathml/presentation-markup/fractions/frac-linethickness-001.html