Screen readers read an image's `alt` attribute and the figure caption,
both of which come from the same source in pandoc. The figure caption is
hidden from screen readers with the `aria-hidden` attribute. This
improves accessibility.
For HTML4, where `aria-hidden` is not allowed, pandoc still uses an
empty `alt` attribute to avoid duplicate contents.
Closes: #6491
The default is `-raw_tex`, so no raw tex should result
unless we explicitly say `+raw_tex`. Previously some
raw commands did make it through.
Closes#4527.
A figure with two subfigures turns into two pandoc
figures; the subcaptions are used and the main caption
ignored, unless there are no subcaptions.
Closes#3577.
