Deprecate/Remove mlabeledtr

[fred-wang]

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

mlabeledtr is supposed to allow equation labels that you can refer easily elsewhere for example:

<p>blah blah</p>

x + y = z                 (5.1)

<p>blah blah see <a href="#id">(5.1)</a> blah blah</p>

MathML3's suggestion is to implement it via a MathML <mtable> via some special <mlabeledtr> row (i.e. <mtr> + some extra label cell). I guess this makes sense in the old "standalone spec" paradigm when each MathML formula is just considered an isolated document, so that specialized math editing/rendering/AT tools can properly handle it. But that does not seem very useful in the Web context and it duplicates existing features: You could just use HTML/CSS layout to produce this labelling effect, just like one writes "Figure 1" in HTML/CSS for images (even SVG ones). LaTeXML or Wikipedia do that (e.g. https://en.wikipedia.org/wiki/Fourier_transform#Definition ).

Additionally, the exact positioning of the label is a bit obscure in MathML3 and would have to be rewritten (maybe part of a general tabular math revamp for CSS) to be compatible with browser layout, implementable and testable. It's not implemented in any browsers and AFAIK there is not any plan to do it in the short to medium term. At this point, it does not seem a feature that meets the criteria to enter in MathML Core IMHO.

Currently MathML Core just treats <mlabeledtr> as an <mtr> with display: none label ( https://mathml-refresh.github.io/mathml-core/#tables ) but it is a bit confusing/misleading out of context and if you are only familiar with HTML tables.

Hence the proposal is either to:
(1) Remove <mlabeledtr> from core for now (my personal preference)
(2) Keep the fallback in core but add some rationale (backward compatibility?, allow users to override the default behavior?)

Last but not least, there is also the usual question of whether we want to deprecate/remove it from the full MathML spec and write polyfills in https://github.com/mathml-refresh/mathml-polyfills.

cc' @bkardell

[davidcarlisle]

I would drop it from core and keep it in full, in a css world the extra label "column" makes things confusing , but in other contexts having an explicit source markup for labels is useful, and dropping the mlabeledtr in any transformation to a document matching the the core spec is likely to be simple enough.

[NSoiffer]

Based on survey results so far, none of the converters generate it, so usage is likely very low. That argues very strongly that it shouldn't be part of core.

The main reason for having this is to make sure the baseline of the label aligns with the baseline of the equation it is labeling. This property is very important to publishers. How can this be done, especially with a multi-line equation (often labeled as 6.1 (a), 6.1 (b), etc?

[pdfion]

As usual, in my view, the point of this construct was to make a close association of a label that might later be referenced with a piece of a displayed formula, so an attempt to allow tying some mathematically significant things together, rather than just to allow a presentation with a formula number, say. If it isn't widely used at all, then remove it from Core by all means. But was not introduced entirely for page layout, though baseline alignment helps provide a good visual cue for an association. It might be argued, I suppose, that other ways of giving document parts IDs are readily available.

[bkardell]

it seems that we have idrefs and labels and links and all sorts of things in the platform that we can use toward these purposes. If it feels like it is replicating something that could be done with the existing platform, it definitely seems like we should either cut it, or find a way to express its very specific implementation directly in known terms. If there is not legacy content, I would prefer to drop it.

[NSoiffer]

I was tasked with getting MathJax stats on this if there were any. I asked, and they don't have any stats on any usage features.

As I mentioned earlier, the point is to get the baseline of the label to align with the baseline of the equation it is labeling. If that can be done in HTML/CSS, then let's drop this from core. To be concrete:

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mtable>
  <mlabeledtr id='e-is-m-c-square'>
    <mtd>
      <mtext> (2.1) </mtext>
    </mtd>
    <mtd>
     <mrow>
       <mi>E</mi>
       <mo>=</mo>
       <mrow>
        <mi>m</mi>
        <mo>&#x2062;<!--INVISIBLE TIMES--></mo>
        <msup>
         <mi>c</mi>
         <mn>2</mn>
        </msup>
       </mrow>
     </mrow>
    </mtd>
  </mlabeledtr>
  <mlabeledtr id='e-is-m-c-square'>
    <mtd>
      <mtext> (2.2) </mtext>
    </mtd>
    <mtd>
     <mrow>
       <mi>V</mi>
       <mo>=</mo>
       <mfrac>
         <mrow>
           <mi>l</mi>
           <mi>w</mi>
           <mi>h</mi>
        </mrow>
        <mn>3</mn>
       </mfrac>
     </mrow>
    </mtd>
  </mlabeledtr>
</mtable>
</math>

Should align the baselines as in

A better example probably would have been something like

   y = ....           label 1
     = ...            label 2
     = ...            label 3

where the exprs have different heights/depths

[NSoiffer]

I scraped 50 ebooks written in PreTeXt. Most are college level textbooks, with some aimed at lower level subjects. PreTeXt uses TeX for the Math and the math is converted to MathML by MathJax. In those 50 books, there are 449,564 expressions, with 26% of them being trivial expressions (a number, identifier, operator, or text). Relevant to this discussion, there were 28,238 mtables, with 69,738 mtrs and 1,405 mlabeledtrs. That's about 2% of the rows. So it does have some use in practice.