Add alttext at math elements

[rgaiacs]

MathML math accepts an alttext attribute that "provides a textual alternative as a fall-back for user agents that do not support embedded MathML or images." This attribute isn't present at the moment:

$ pandoc --version
pandoc 1.13
$ cat math.html 
<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>a</mi><mn>2</mn></msup><mo>+</mo><msup><mi>b</mi><mn>2</mn></msup><mo>=</mo><msup><mi>c</mi><mn>2</mn></msup></mrow></math></p>
$ cat math.md 
$a^2 + b^2 = c^2$
$ pandoc -f markdown -t html5 --mathml -o math.html math.md
$ cat math.html                                            
<p><math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>a</mi><mn>2</mn></msup><mo>+</mo><msup><mi>b</mi><mn>2</mn></msup><mo>=</mo><msup><mi>c</mi><mn>2</mn></msup></mrow></math></p>

Will be nice have this alttext with the LaTeX source of the equation or use MathML annotation for it.

@fred-wang Do you like to say something?

[fred-wang]

MathML annotation is best for copy & paste (MathJax, https://addons.mozilla.org/en-US/firefox/addon/mathml-copy/ etc). alttext is only a fallback in situation when the MathML rendering is not possible.

[mpickering]

I think you are meant to put this sort of information in the semantics element.

This wouldn't be hard to do, I just avoided doing so the first time to make the output less cluttered.

[mpickering]

From my dev build

> .cabal-sandbox/bin/pandoc -f markdown -t html5 --mathml -o test.html
$a^2 + b^2 = c^2$
> cat test.html 
<p><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>a</mi><mn>2</mn></msup><mo>+</mo><msup><mi>b</mi><mn>2</mn></msup><mo>=</mo><msup><mi>c</mi><mn>2</mn></msup></mrow><annotation encoding="application/x-tex">a^2 + b^2 = c^2</annotation></semantics></math></p>

This works with @fred-wang's extension.

[jgm]

Sounds like a good idea, but it's not entirely simple. The mathml
writer in texmath can't do it, because its input is a parse tree
(so it doesn't know about the latex source). So it would need to be
inserted in pandoc, or maybe texmath could export a function to inject
this information into a mathml xml element.

+++ mpickering [Sep 25 14 06:46 ]:

I think you are meant to put this sort of information in the semantics element.

This wouldn't be hard to do, I just avoided doing so the first time to make the output less cluttered.

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[jgm]

Looks good.

+++ mpickering [Sep 25 14 07:28 ]:

From my dev build

> .cabal-sandbox/bin/pandoc -f markdown -t html5 --mathml -o test.html
$a^2 + b^2 = c^2$
> cat test.html
<p><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>a</mi><mn>2</mn></msup><mo>+</mo><msup><mi>b</mi><mn>2</mn></msup><mo>=</mo><msup><mi>c</mi><mn>2</mn></msup></mrow><annotation encoding="application/x-tex">a^2 + b^2 = c^2</annotation></semantics></math></p>

This works with @fred-wang's extension.

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