Pass through aria- attributes to HTML5.

Also document addition of data- prefix to unknown attributes.

Closes #5646.

MANUAL.txt

@@ -3967,10 +3967,11 @@ Attributes can be set on links and images:
 (This syntax is compatible with [PHP Markdown Extra] when only `#id`
 and `.class` are used.)
 
-For HTML and EPUB, all attributes except `width` and `height` (but
-including `srcset` and `sizes`) are passed through as is. The other
-writers ignore attributes that are not supported by their output
-format.
+For HTML and EPUB, all known HTML5 attributes except `width` and
+`height` (but including `srcset` and `sizes`) are passed through
+as is. Unknown attributes are passed through as custom
+attributes, with `data-` prepended. The other writers ignore
+attributes that are not specifically supported by their output format.
 
 The `width` and `height` attributes on images are treated specially. When
 used without a unit, the unit is assumed to be pixels. However, any of

src/Text/Pandoc/Writers/HTML.hs

@@ -621,6 +621,7 @@ toAttrs kvs = do
                   if x `Set.member` (html5Attributes <> rdfaAttributes)
                      || ':' `elem` x -- e.g. epub: namespace
                      || "data-" `isPrefixOf` x
+                     || "aria-" `isPrefixOf` x
                      then Just $ customAttribute (fromString x) (toValue y)
                      else Just $ customAttribute (fromString ("data-" ++ x))
                                   (toValue y)

test/command/5646.md

@@ -0,0 +1,8 @@
+```
+% pandoc -t html5 -f markdown
+![test](foo){aria-describedby="barbaz"}
+^D
+<figure>
+<img src="foo" aria-describedby="barbaz" alt="" /><figcaption>test</figcaption>
+</figure>
+```