Math role edits (Was: MathML - and action 1494.)

Here's the informative note I wrote for Action-1494. Any concerns, objections, or suggestions?

> NOTE
> Browsers that support native implementations of MathML are able to provide a more robust, accessible math experience than can be accomplished with plain text approximations of math. Some rendering engines have close integration with screen readers that allow spacial touch exploration of the formula and refreshable braille display output in the Nemeth Braille <http://rawgit.com/w3c/aria/master/spec/aria.html#dfn-nemeth-braille> format. This level of integration is not supported with images of mathematical formulas, even if the author provides a plain text approximation.
> 
> At the time of this writing, some mainstream browsers do not support MathML natively, and must be retrofit using a JavaScript polyfill library. When authoring math content, use native MathML wherever possible, and test thoroughly. Use a polyfill library or provide a fallback image with a text alternative approximation if necessary.
> 

In order to address additional comments from Rich and others, I massaged some of the ARIA 1.0 text as well. For example, it now mentions "native implementations and polyfill libraries" instead of indicating math would rely on a "plug-in."

http://rawgit.com/w3c/aria/master/spec/aria.html#math
https://github.com/w3c/aria/commit/de79388c1cd65e2fc07d8789ae546b16f87b3df2

I don't believe the text massaging resulted in any significant normative changes, but please review the diff or compare the current WD to the ED if you desire. I did include a new image fallback example with some author suggestions (a normative MAY), and an editorial note about screen reader verbosity.
> Plain HTML or Polyfill DOM Result of the MathML Quadratic Formula§ <http://rawgit.com/w3c/aria/master/spec/aria.html#desc-math>
> If a rendering engine does not support a native math format such as MathML, authors may use JavaScript to downgrade the content to a format the browser can display, such as this HTML image using a data URI and plain text alternative.
> 
> EXAMPLE 9
> <img role="math" src="..." alt="x=(−b±√(b²−4ac))÷2a">
> Editor's note: Might need an RFC-2119 "should" requirement here to encourage AT to speak math approximations with high punctuation verbosity. Otherwise ambiguous characters like a forward slash (/) may not be spoken even when intended to be used interchangeably with the division sign character (÷).
> 

We discussed this topic in the ARIA call this morning and I believe everyone on the call was generally in agreement. If there are concrete concerns, please mention the relevant spec text and suggest changes where appropriate.

Thanks,
James Craig

-- 
Indifference towards people and the reality in 
which they live is actually the one and only 
cardinal sin in design. — Dieter Rams