Add support for dpub-aria in HTML output

[link2xt]

See jgm/pandoc#4213

There is no benefit yet, and Text::Amuse HTML output is not going to be processed by machine probably (there is a .muse source available, after all), but it might improve accessibility and help search engines index HTML output.

[melmothx]

Agreed, not a big deal and totally worth the effort.

[melmothx]

Actually, this is a no-go. Validation barfs everywhere.

https://validator.w3.org/
there is no attribute "role"

  <h2 id="toc2" role="doc-part">Prova 2</h2>

http://validator.idpf.org/

Error while parsing file 'attribute "role" not allowed here; expected attribute "class", "dir", "lang", "style", "title" or "xml:lang"'.

[link2xt]

I think "role" should be attached to "section", "aside", "p" etc., something that actually holds contents, not h1, h2, ...

See https://stackoverflow.com/questions/4170356/what-values-can-the-html5-xhtml-role-attribute-have
Formal specification of HTML5 explicitly permits it: https://w3c.github.io/html/dom.html#wai-aria

[melmothx]

Well, the problem is that the HTML in the epub (version 2) is XHTML strict, not HTML5. Matter needs to be digged.

[melmothx]

as far as I can see, EPUB would need to migrate to 3.0 (with extra steps for backcompat) and HTML to HTML5.