Expose MathML for accessibility

Created by: jcsteh

In order for screen readers and other assistive technology (AT) products to provide rich access to math content, they need to be able to access the raw math in a standard way. Providing alternate text is not sufficient, since although this does allow for better spoken math, it does not provide for braille, interactive exploration, etc. It seems MathML has become the dominant standard for exposing math content and MathJax uses MathML internally, so it'd make sense to expose it as MathML.

You can already use the toMathML extension to access MathML for a MathJax tree. The problem is that this requires JavaScript calls and an AT does not always have the ability to execute arbitrary JavaScript on the page. (For example, in Firefox, this isn't possible without a browser extension or Greasemonkey script.) There needs to be some way to obtain MathML from MathJax that does not require JavaScript calls.

One solution is to expose the MathML via an attribute; e.g. data-mathml. This is pretty inelegant and non-standard, but it does solve the problem. I've already implemented experimental support for this in NVDA's (not yet released) math support.

If exposing an attribute like this automatically is considered too expensive, we might be able to come up with a solution using an off-screen button or similar which exposes the attribute when activated. However, this is even less elegant and is more error prone, so I'd hope to avoid something like this if possible.

/cc @pkra