Improve our accessibility metadata

[robinwhittleton]

Some suggestions from @acabal that I’ll investigate:

1. Add accessibility compliance statements to onix.xml (see https://onix-codelists.io/codelist/196)
2. Make sure that MathML accessibility compliance notes are stripped from compatible builds
3. Review https://www.w3.org/TR/epub-a11y-11/ to see if there’s a better way of presenting our ePub accessibility compliance e.g.:

<meta
     property="dcterms:conformsTo"
     id="conf">
    EPUB Accessibility 1.1 - WCAG 2.1 Level AA
</meta>

[robinwhittleton]

This PR goes some way towards updating the accessibility metadata, but some other potential changes to discuss as well:

• ONIX accessibility number 96 is “Publisher’s web page for detailed accessibility information”. We currently don’t have an accessibility page on standardebooks.org but it wouldn’t be hard to add one. Would you like me to do that?
• The endnotes links are somewhat cryptic. One thing we could do is change <a epub:type="noteref">1</a> to <a aria-label="View endnote 1" epub:type="noteref">1</a> or something like that. As part of this we’d also update the link back to the book from the endnote to include an aria-label too. This obviously requires a little more work from our producers, but nothing that’s different from what they’re doing already.
• I’ll also re-review standardebooks.org for accessibility, as getting the books is as much a part of the process as reading the books.

Thoughts?

[acabal]

Great, thanks.

Not sure about aria-label on endnotes. Presumably if a reading system sees that this is a noteref, it should announce it as such. I suppose aria-label makes it explicit but it's also more work for us and could override an otherwise preferred ereader default announcement.

Sure, you can review the website too!

[robinwhittleton]

OK, I’ll put together an accessibility page over the next few days. Once that’s done then there’ll be another ONIX PR: up to you whether you want to do a corpus update now and another after, or to wait for that.

[robinwhittleton]

I’ve added the MathML ONIX feature to all the books that needed it now.

[acabal]

Great, thanks!