[PATCH] html export: remove W3C warning, improve accessibility

[Timothy]

Hi All,

Following on from my comments /ages/ ago, I have done some more work to
improve the HTML export. I intend to push it if I don't hear any
complaints. Please let me know if anything looks dodgy, or if anything
should also be improved with this patch :)

This patch:
- Resolves a W3C warning about type="text/css" in <style>
- Removes a docstring reference to IE8- (urgh!)
- Adds some ARIA roles to improve accessibility (let me know if you spot
anything else that could benefit from them being added).

Running the default html export through the W3C validator with my
massive Org-based config now results in nothing other than complaints
about dodgy unicode characters (not Org's fault) :)

--
Timothy

0001-ox-html-Resolve-W3C-warning-improve-a11y.patch
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